Wednesday, June 3, 2020

Sharing a single screen in Slack for Linux

I have a bunch of monitors, and for whatever reason, Slack for Linux refuses to let me limit my screen sharing to a single monitor or application.  This means that if I try to share my screen on a call, no one can see or read anything because they just see a giant, wide view of three monitors' worth of pixels crammed into their Slack window (typically only one monitor wide).

In my experience, disabling monitors/displays is just not worth it; I'll have to spend too much time getting everything set back up correctly afterward, and that's really inconvenient and annoying.

The solution that I've landed on is Xephyr; Xephyr runs a second X11 server inside a new window, so when I need to get on a call where I'll have to share my screen, I simply:
  1. Launch a new Xephyr display.
  2. Close Slack.
  3. Open Slack on the Xephyr display.
  4. Open whatever else I'll need to share in the Xephyr display, typically a web browser or a terminal.
  5. Get on the Slack call and share my "screen".
Some small details:
  • You'll need to open Xephyr with the resolution that you want; given window decorations and such, you may need to play around with this a bit.  Once you find out what works, put it in a script.
  • In order to resize windows in Xephyr, it'll need to be running a window manager.  I struggled to get any "startx"-related things working, but I found that "twm" worked well enough for my purposes.
  • Some applications, such as Chrome, won't open on two displays at the same time.  I just open a different browser in my Xephyr display (for example, I use "google-chrome" normally and "chromium-browser" in Xephyr), but you can also run Chrome using a different profile directory and it'll run in the other display.
Install Xephyr and TWM:
sudo apt install xserver-xephyr twm

Run Zephyr, Slack, and Chromium:
# Launch Xephyr and create display ":1".
Xephyr -ac -noreset -screen 1920x1000 :1 &
# Start a window manager in Xephyr.
DISPLAY=:1 twm &>/dev/null &
# Open Slack in Xephyr.
DISPLAY=:1 slack &>/dev/null &
# Open Chromium in Xephyr.
DISPLAY=:1 chromium-browser &>/dev/null &

It's kind of dirty, but it works extremely well, and I don't have to worry about messing with my monitor setup when I need to give a presentation.

Edit: an earlier version of this post used "Xephyr -bc -ac -noreset -screen 1920x1000 :1 &" for the Xephyr command; I can't get this to work with "-bc" anymore; I must have copied the wrong command when I published the post.

Wednesday, January 29, 2020

Unit-testing reCAPTCHA v2 and v3 in Go

I recently worked on a project where we allowed new users to sign up for our system with a form.  A new user would need to provide us with her name, her e-mail address, and a password.  In order to prevent spamming, we used reCAPTCHA v3, and so that meant that we also submitted a reCAPTCHA token along with the rest of the new-user data.

Unit-testing the sign-up process was fairly simple if we turned off the reCAPTCHA requirement, but the weakest link in the whole process is the one part that we could not control: reCAPTCHA.  It would be foolish not to have test coverage around the reCAPTCHA workflow.

So, how do you unit-test reCAPTCHA?

Focus: Server-side testing

For the purposes of this post, I'm going to be focusing on testing reCAPTCHA on the server side.  This means that I'm not concerned with validating that users acted like humans fiddling around on a website.  Instead, I'm concerned with what our sign-up endpoint does when it receives valid and invalid reCAPTCHA tokens.

reCAPTCHA in Go

There are a variety of Go package to provide reCAPTCHA support; however, only one of them (1) has support for Go modules, and (2) has support for unit testing built in:

For docs, see:

When you create a new reCAPTCHA site, you're given public and private keys (short little strings, nothing huge).  The public key is used on the client side when you make your connection to the reCAPTCHA API, and a response token is provided back.  The private key is used on the server side to connect to the reCAPTCHA API and validate the response token.

Since the client side will likely be a line or two of Javascript that generates a token, our server-side work will be focused on validating that token.

Assuming that the newly generated token is "NEW_TOKEN" and that the private key is "YOUR_PRIVATE_KEY", then this is all you have to do in order to validate that token:

import "github.com/tekkamanendless/go-recaptcha"

// ...

recaptchaVerifier := recaptcha.New("YOUR_PRIVATE_KEY")
success, err := recaptchaVerifier.Verify("NEW_TOKEN")
if err != nil {
   // Fail with some 500-level error about not being able to verify the token
}


if !success {
   // Fail with some 400-level error about not being a human
}


// The token is valid!


And that's it!  All we really care about is whether or not it worked.

Unit testing

tekkamanendless/go-recaptcha includes a package called "recaptchatest" that provides a fake reCAPTCHA API running as an httptest.Server instance.  This server simulates enough of the reCAPTCHA API to let you do the kinds of testing that you need to.

Just like the actual reCAPTCHA service, you can create multiple "sites" on the test server.  Each site will have a public and private key, and you can call the NewResponseToken method of a site to have that site generate a valid token for that site.

In terms of design, you'll set up the test server, the test site, and the valid token in advance of your test.  When you create your Recaptcha instance with the test site's private key, all you have to do is set the VerifyEndpoint property of that instance to point to the test server (otherwise, it would try to talk to the real reCAPTCHA API and fail).

Here's a simple example:

import (
   "github.com/tekkamanendless/go-recaptcha" 
   "github.com/tekkamanendless/go-recaptcha/recaptchatest"
)

// ...

// Create a new reCAPTCHA test server, site, and valid token before the main test.
testServer := recaptchatest.NewServer()
defer testServer.Close()

site := testServer.NewSite()
token := site.NewResponseToken()

// Create the reCAPTCHA verifier with the site's private key.
recaptchaVerifier := recaptcha.New(site.PrivateKey)

// Override the endpoint so that it uses the test server.
recaptchaVerifier.VerifyEndpoint = testServer.VerifyEndpoint()

// Run your test.

// ...

// Validate that the reCAPTCHA token is good.
success, err := recaptchaVerifier.Verify(token)
assert.Nil(t, err)
assert.True(t, success)

The recaptchatest test server doesn't do too much that's fancy, but it will properly return a failure if the same token is verified twice or if the token is too old.  It also has some functions to let you tweak the token properties so you don't have to wait around for 2 minutes for a token to age out; you can make one that's already too old (see Site.GenerateToken for more information).

Monday, October 14, 2019

Encrypt your /home directory using LUKS and a spare disk

Every year or two, I rotate a drive out of my NAS.  My most recent rotation yielded me with a spare 1TB SSD.  My main machine only had a 250GB SSD, so I figured that I'd just replace my /home directory with a mountpoint on that new disk, giving my lots of space for video editing and such, since I no longer had the room to deal with my GoPro footage.

My general thought process was as follows:

  1. I don't want to mess too much with my system.
  2. I don't want to clone my whole system onto the new drive.
  3. I want to encrypt my personal data.
  4. I don't really care about encrypting the entire OS.
I had originally looked into some other encryption options, such as encrypting each user's home directory separately, but even in the year 2019 there seemed to be too much drama dealing with that (anytime that I need to make a PAM change, it's a bad day).  Using LUKS, the disk (well, partition) is encrypted, so everything kind of comes for free after that.

If you register the partition in /etc/crypttab, your machine will prompt you for the decryption key when it boots (at least Kubuntu 18.04 does).

One other thing: dealing with encrypted data may be slow if your processor doesn't support AES encryption.  Do a quick check and make sure that "aes" is listed under "Flags":
lscpu;
If "aes" is there, then you're good to go.  If not, then maybe run some tests to see how much CPU overhead disk operations use on LUKS (you can follow this guide, but stop before "Home setup, phase 2", and see if your overhead is acceptable).

The plan

  1. Luks Setup
    1. Format the new disk with a single partition.
    2. Set up LUKS on that partition.
    3. Back up the LUKS header data.
  2. Home setup, phase 1
    1. Copy everything in /home to the new partition.
    2. Update /etc/crypttab.
    3. Update /etc/fstab using a test directory.
    4. Reboot.
    5. Test.
  3. Home setup, phase 2
    1. Update /etc/fstab using the /home directory.
    2. Reboot.
    3. Test.

LUKS setup

Wipe the new disk and make a single partition.  For the remainder of this post, I'll be assuming that the partition is /dev/sdx1.

Install "cryptsetup".
sudo apt install cryptsetup; 
Set up LUKS on the partition.  You'll need to give it a passphrase.  I recommend something that's easy to type, like a series of four random words, but you do you).  You'll have to type this passphrase every time that you boot your machine up.
sudo cryptsetup --verify-passphrase luksFormat /dev/sdx1;
Once that's done, you can give it some more (up to 8) passphrases.  This may be helpful if you want to have other people access the disk, or if you just want to have some backups, just in case.  If there are multiple passphrases, any one of them will work fine; you don't need to have multiple on hand.
sudo cryptsetup --verify-passphrase luksAddKey /dev/sdx1;
The next step is to "open" the partition.  The last argument ("encrypted-home") is the name to use for the partition that will appear under "/dev/mapper".
sudo cryptsetup luksOpen /dev/sdx1 encrypted-home;
At this point, everything is set up and ready ready.  Confirm that with the "status" command.
sudo cryptsetup status encrypted-home;
Back up the LUKS header data.  If this information gets corrupted on the disk, then there is no way to recover your data.  Note that if you recover data using the header backup, then the passphrases will be the ones in the header backup, not whatever was on the disk at the time of the recovery.
sudo cryptsetup luksHeaderBackup /dev/sdx1 --header-backup-file /root/luks.encrypted-home.header;
I put mine in the /root folder (which will not be on the encrypted home partition), and I also backed it up to Google Drive.  Remember, if you add, change, or delete passphrases, you'll want to do make another backup (otherwise, those changes won't be present during a restoration operation).

If you're really hardcore, fill up the partition with random data so that no part of it looks special.  Remember, the whole point of encryption is to make it so that whatever you wrote just ends up looking random, so writing a bunch of zeros with "dd" will do the trick:
sudo dd if=/dev/zero of=/dev/mapper/encrypted-home;
Before you can do anything with it, you'll need to format the partition.  I used EXT4 because everything else on this machine is EXT4.
sudo mkfs.ext4 /dev/mapper/encrypted-home;

Home setup, phase 1

Once the LUKS partition is all set up, the next set of steps is just a careful copy operation, tweaking a couple /etc files, and verifying that everything worked.

The safest thing to do would be to switch to a live CD here so that you're guaranteed to not be messing with your /home directory, but I just logged out of my window manager and did the next set of steps in the ctrl+alt+f2 terminal.  Again, you do you.

Mount the encrypted home directory somewhere where we can access it.
sudo mkdir /mnt/encrypted-home; 
sudo mount /dev/mapper-encrypted-home /mnt/encrypted-home;
Copy over everything in /home.  This could take a while.
sudo cp -a /home/. /mnt/encrypted-home/;
Make sure that /mnt/encrypted-home contains the home folders of your users.

Set up /etc/crypttab.  The format is:
${/dev/mapper name} UUID="${disk uuid}" none luks
In our case, the /dev/mapper name is going to be "encrypted-home".  To find the UUID, run:
sudo blkid /dev/sdx1;
So, in my particular case, /etc/crypttab looks like:
encrypted-home UUID="5e01cb97-ceed-40da-aec4-5f75b025ed4a" none luks
Finally, tell /etc/fstab to mount the partition to our /mnt/encrypted-home directory.  We don't want to clobber /home until we know that everything works.

Update /etc/fstab and add:
/dev/mapper/encrypted-home /mnt/encrypted-home ext4 defaults 0 0
Reboot your machine.

When it comes back up, it should ask you for the passphrase for the encrypted-home partition.  Give it one of the passphrases that you set up.

Log in and check /mnt/encrypted-home.  As long as everything's in there that's supposed to be in there (that is, all of your /home data), then phase 1 is complete.

Home setup, phase 2

Now that we know everything works, the next step is to clean up your actual /home directory and then tell /etc/fstab to mount /dev/mapper/encrypted-home at /home.

I didn't want to completely purge my /home directory; instead, I deleted everything large and/or personal in there (leaving my bash profile, some app settings, etc.).  This way, if my new disk failed or if I wanted to use my computer without it for some reason, then I'd at least have normal, functioning user accounts.  Again, you do you.  I've screwed up enough stuff in my time to like to have somewhat nice failback scenarios ready to go.

Update /etc/fstab and change /dev/mapper/encrypted-home line to mount to /home.
/dev/mapper/encrypted-home /home ext4 defaults 0 0
Reboot.

When it comes back up, it should ask you for the passphrase for the encrypted-home partition.  Give it one of the passphrases that you set up.

Log in.  You should now be using an encrypted home directory.  Yay.

To confirm, check your mountpoints:
mount | grep /home
You should see something like:
/dev/mapper/encrypted-home on /home type ext4 (rw,relatime,data=ordered)
Now that everything's working, you can get rid of "/mnt/encrypted-home"; we're not using it anymore.
sudo rmdir /mnt/encrypted-home;

Friday, March 23, 2018

Fix for when Chrome stops making screen updates

My desktop environment is KDE, and I use Chrome for my browser.  At any given time, I'll have 2-5 windows with 10-40 tabs each.  However, every once in a while (usually once every week or so), the rendering of Chrome will freeze.  That is, the entire window will remain frozen (visually), but clicks and everything else go through fine (you just can't see the results).  Changing my window focus (switching to a different window, opening the "K" menu, etc.) usually causes a single render, but that doesn't help with actually interacting with a (visually) frozen Chrome window.

Closing Chrome and opening it back up works, but that's really inconvenient.

I'm still not sure why this happens, but I do have a quick (and convenient) fix: change your compositor (and then change it back).  Why does this work?  I'm not sure, but since it's obviously a rendering problem, making a rendering change makes sense.

Step by step:

  1. Open "System Settings".
  2. Open "Display and Monitor".
  3. Go to "Compositor".
  4. Change "Rendering backend" from whatever it is to something else (usually "OpenGL 3.1" to "OpenGL 2.0" or vice versa).
  5. Click "Apply".
This always solves the problem for me.  You can even switch it back to the original value after you hit "Apply" the first time.

Monday, October 16, 2017

Hidden dependencies with Gerrit and Bazel

This is going to be very short.

I was trying to build the "go-import" plugin for Gerrit (because apparently no one wants to build it and publish it), and I spent the past four hours trying to get "bazel build ..." to actually run.

If you're getting errors like these:
root@25f962af2a00:/gerrit# bazel fetch //...
ERROR: /gerrit/lib/jetty/BUILD:1:1: no such package '@jetty_servlet//jar': Argument 0 of execute is neither a path nor a string. and referenced by '//lib/jetty:servlet'
ERROR: /gerrit/lib/codemirror/BUILD:11:1: no such package '@codemirror_minified//jar': Argument 0 of execute is neither a path nor a string. and referenced by '//lib/codemirror:mode_q_r'
ERROR: /gerrit/lib/codemirror/BUILD:11:1: no such package '@codemirror_minified//jar': Argument 0 of execute is neither a path nor a string. and referenced by '//lib/codemirror:theme_solarized_r'
ERROR: /gerrit/lib/codemirror/BUILD:11:1: no such package '@codemirror_original//jar': Argument 0 of execute is neither a path nor a string. and referenced by '//lib/codemirror:theme_elegant'
ERROR: Evaluation of query "deps(//...)" failed: errors were encountered while computing transitive closure
Building: no action running

root@25f962af2a00:/gerrit# bazel build --verbose_failures plugins/go-import:go-import

ERROR: /gerrit/lib/BUILD:176:1: no such package '@jsr305//jar': Argument 0 of execute is neither a path nor a string. and referenced by '//lib:jsr305'
ERROR: Analysis of target '//plugins/go-import:go-import' failed; build aborted

Then you probably also need these packages:

  1. nodejs
  2. npm
  3. zip
I was building on a Docker container, and it didn't have any of those.

The "Argument 0 of execute" line appears to be referring to some internal call to "execute" that looks something like "execute("npm",...)" or "execute("zip",...").

Anyway, that instantly fixed my problem.

Friday, March 17, 2017

Shrinking a disk to migrate to a smaller SSD

I have finally migrated all of my storage to solid state drives (SSDs).  For my older machines, the 256GB SSDs were bigger than their normal hard drives, so Clonezilla worked perfectly well.  The very last holdout was my 2TB Proxmox server, since my largest SSD was 480GB.

Since I was only using about 120GB of space on that disk, my plan was to shrink the filesystems, then shrink the partitions, and then clone the disk with Clonezilla.

tl;dr both Clonezilla and grub can be jerks.

Even after shrinking everything down, Clonezilla refused to clone the disk because the SSD was smaller than the original (even though all of the partitions would fit on the SSD with plenty of room to spare).  Clonezilla did let me clone a partition at a time, so I did that, but the disk wouldn't boot.  To make a long story short, you need to make sure that you run "update-grub" and "grub-install" after pulling a disk-switcheroo stunt.  And yes, to do that, you'll need to use a live OS to chroot in (and yes, you'll need to mount "/dev", etc.) and run those commands.

Just that paragraph above would have saved me about six hours of time, so there it is, for the next person who is about to embark on a foolish and painful journey.

Shrink a disk

My Proxmox host came with a 2TB spinner disk, and I foolishly decided to use it instead of putting in a 256GB SSD on day one.  I didn't feel like reinstalling Proxmox and setting all of my stuff up again, and I certainly didn't want to find out that I didn't back up some crucial file or couldn't remember the proper config; I just wanted my Proxmox disk migrated over to SSD, no drama.

Remember, you aren't actually shrinking the disk itself; rather, you're reducing the allocated space on that disk by shrinking your filesystems and partitions (and, if you're using LVM, your logical volumes and physical volumes, too).

So, the goal is: shrink all of your partitions down to a size that they can be copied over to a smaller disk.
Remember, you don't have to shrink all of your partitions, on the big ones.  In my case, I had a 1MB partition, a 500MB partition, and a 1.9TB partition.  Obviously, I left the first two alone and shrunk the third one.

First: Shrink whatever's on the partition

How easy this step is depends on if you're using LVM or not.  If you are not using LVM and just have a normal filesystem on the partition, then all you need to do is resize the filesystem to a smaller size (obviously, you have to have less space in use than the new size).

If you are using LVM, then your partition is full of LVM stuff (namely, all of the data needed to make the logical volumes (LVs) that reside on that partition (which is the physical volume, or PV)).
Note: you cannot mess with a currently-mounted filesystem.  If you need to resize your root filesystem, then you'll have to boot into a live OS and do it from there.
(From here on, we'll use "sda" as the original disk and "sdb" as the new SSD.)

 Option 1: Filesystem

Shrink the filesystem using the appropriate tool for your filesystem.  For example, let's assume that we want to shrink "/dev/sda3" down to 50GB.  You could do this with an EXT4 filesystem via:
resize2fs -p /dev/sda3 50G

Option 2: LVM

Shrink whatever filesystems are on the logical volumes that are on the physical volume that you want to shrink.  For example, let's assume that we want to shrink the "your-lv" volume down to 50GB.
resize2fs -p /dev/your-vg/your-lv 50G

Then, shrink the logical volume to match:
lvresize /dev/your-vg/your-lv --size 50G

Now that the logical volume has been shrunk, you should be able to see free space in its physical volume.  Run "pvs" and look at the "PFree" column; that's how much free space you have.

Now shrink the physical volume:
pvresize /dev/sda3 --setphysicalvolumesize 50G

Second: Shrink the partition

Be very, very sure of what you want the new size of your partition to be.  Very sure.  The surest that you've ever been in your life.

Then, use "parted" or "fdisk" to (1) delete the partition, and then (2) add a new partition that's smaller.  The new partition must start on the exact same spot that the old one did.  Make sure that the partition type is set appropriately ("Linux" or "Linux LVM", etc.).  Basically, the new one should look exactly like the old one, but with the ending position smaller.

Clone a disk

Use Clonezilla (or any other live OS, for that matter) to clone the partitions to the new disk.  With Clonezilla, I still had to make the partitions on the new disk myself, so while you're there, you might as well just "dd" the contents of each partition over.

Create the new partitions and copy the data

On the new disk, create partitions that will correspond with those on the old disk.  You can create them with the same size, or bigger.  I created my tiny partitions with the same sizes (1MB and 500MB), but I created my third partition to take up the rest of the SSD.

Copy the data over using whatever tool makes you happy; I like "dd" because it's easy.  Be very, very careful about which disk is the source disk (input file, or "if") and which is the target disk (output file, or "of").  For example:
dd if=/dev/sda1 of=/dev/sdb1 bs=20M
dd if=/dev/sda2 of=/dev/sdb2 bs=20M
dd if=/dev/sda3 of=/dev/sdb3 bs=20M

If you create larger partitions on the new disk, everything will still work just fine; the filesystems or LVM physical volumes simply will have the original sizes.  Once everything is copied over, you can simply expand the filesystem (or, for LVM, expand the physical volume, then the logical volume, and then the physical volume) to take advantage of the larger partition.

Remove the original disk

Power off and remove the original disk; you're done with it anyway.

If you're using LVM, that original disk will only get in the way, since it'll get confused about having double the number of everything and won't set up your volumes correctly.

(From here on, we'll use "sda" as the new SSD since we've removed the original disk.)

Bonus: expand your filesystems

Now that everything's copied over to the new disk, you can expand your filesystems to take up the full size of the partition.  You may have to run "fsck" before it'll let you expand the filesystem.

For a normal filesystem, simply use the appropriate tool for your filesystem.  For example, let's assume that we want to grow "/dev/sda3" to take up whatever extra space there might be.  You could do this with an EXT4 filesystem via:
resize2fs /dev/sda3

In the case of LVM, you'll have to expand the physical volume first, then the logical volume, and then the filesystem (essentially, this is the reverse of the process to shrink them).  For example:
pvresize /dev/sda3
lvresize -l +100%FREE /dev/your-vg/your-lv
resize2fs /dev/your-vg/your-lv
Note: the "-l +100%FREE" arguments to "lvresize" tell it to expand to include all ("100%") of the free space available on the physical volume.

Fix grub

I still don't fully grok the changes that UEFI brought to the bootloader, but I do know this: after you pull a disk-switcheroo, grub needs to be updated and reinstalled.

So, using your live OS, mount the root filesystem from your new disk (and any other dependent filesystem, such as "/boot" and "/boot/efi").  Then create bind mounts for "/dev", "/dev/pts", "/sys", "/proc", and "/run", since those are required for grub to install properly.

Finally, chroot in to the root filesystem and run "update-grub" followed by "grub-install" (and give it the path to the new disk):
update-grub
grub-install /dev/sda

Conclusion

That should do it.  Your new disk should boot properly.

For a decent step-by-step guide on shrinking an LVM physical volume (but with the intent of creating another partition), see this article from centoshelp.org.

Live-updating a Java AppEngine project's HTML with Maven

I recently switched all of my Java AppEngine projects to use Maven once the Google updated its Eclipse plugin to support Google Cloud SDK (instead of the AppEngine SDK); see the getting-started with Maven guide.  Google Cloud SDK is much more convenient for me because it gets regular updates via "apt-get" as the "google-cloud-sdk" package, plus it has a nice CLI for working with an app (great for emergency fixes via SSH).

Maven is relatively new to me; I tried it years ago, but it didn't play well with the AppEngine plugin for Eclipse, so I gave up.  Now that it's fully supported, I am very happy with it.  tl;dr is that it's the Java compiler and toolkit that you always thought existed, but didn't.

There is exactly one problem with the new Eclipse plugin for AppEngine/Maven support: the test server doesn't respect any of the plugin settings in my "pom.xml" file.  This means that it won't run on the port that I tell it to run on, etc.

That's not a huge deal because I can just run "mvn appengine:run" to run it perfectly via the command line (and I love my command line).
Aside: the command is actually slightly more involved since AppEngine needs Java 7 (as opposed to Java 9, which is what my desktop runs).
JAVA_HOME=/usr/lib/jvm/java-7-oracle/ mvn appengine:run
This worked perfectly while I was editing my Java source files.  The default "pom.xml" has a line in it that tells eclipse to compile its Java code to the appropriate Maven target directory, which means that the development server will reload itself anytime that, effectively, the Java source code changes.
In case you're wondering, that line is:
<!-- for hot reload of the web application--> <outputDirectory>${project.build.directory}/${project.build.finalName}/WEB-INF/classes</outputDirectory>
However, another time, I had to update the HTML portion of my app, and my changes weren't being applied.  I'd change a file, reload a million times, and nothing would work.  Restarting the development server would work, but that was crazy to me: why would the development server support a hot reload for Java source code changes but not for trivial static files (such as the HTML files)?

I spent hours and hours troubleshooting this very thing, and I finally learned how this whole Maven thing works:
  1. Maven compiles the application (including the Java code) to a "target".
  2. For development purposes, the target is a directory that can be scanned for changes; for production purposes, the target is a "jar" or "war" file, for example.
  3. That "outputDirectory" tag in "pom.xml" tells Eclipse to compile its Java changes to the appropriate target directory, allowing a running development server to hot reload the Java changes.
When the target directory is built, all of the static files are copied to it.  When you edit such a file in Eclipse, you're editing the original file in the "src/main/webapp" folder, not the target folder that the development server is running from.

To solve the problem (that is, to add hot reload for static files), you simply need to mirror whatever change you made in the original folder to the copy in the target folder.

I spent too many hours trying to figure out how to do this in Eclipse and eventually settled on a short bash script that continually monitors the source directory using "inotifywait" for changes and then mirrors those changes in the target directory (as I said, I love my command line).

My solution is now to run "mvn appengine:run" in one terminal and this script in another (it uses "pom.xml" to figure out the target directory):

#!/bin/bash

declare -A commands;
commands[inotifywait]=inotify-tools;
commands[xmlstarlet]=xmlstarlet;

for command in "${!commands[@]}"; do
   if ! which "${command}" &>/dev/null; then
      echo "Could not find command: ${command}";
      echo "Please intall it by running:";
      echo "   sudo apt-get install ${commands[$command]}";
      exit 1;
   fi;
done;

artifactId=$( xmlstarlet sel -N my=http://maven.apache.org/POM/4.0.0 --template --value-of /my:project/my:artifactId pom.xml );
version=$( xmlstarlet sel -N my=http://maven.apache.org/POM/4.0.0 --template --value-of /my:project/my:version pom.xml );

sourceDirectory=src/main/webapp/;
sourceDirectoryLength="${#sourceDirectory}";
targetDirectory=target/"${artifactId}-${version}"/;

echo "Source directory: ${sourceDirectory}";
echo "Target directory: ${targetDirectory}";

inotifywait -m -r src/main/webapp/ -e modify -e moved_to -e moved_from -e create -e delete 2>/dev/null | while read -a line; do
   echo "Line: ${line[@]}";
   fullPath="${line[0]}";
   operation="${line[1]}";
   filename="${line[2]}";
   
   path="${fullPath:$sourceDirectoryLength}";

   echo "Operation: ${operation}";
   echo "   ${path} :: ${filename}";

   case "${operation}" in
      CREATE|MODIFY|MOVED_TO)
         cp -v -a -f "${fullPath}${filename}" "${targetDirectory}${path}${filename}";
         ;;
      DELETE|MOVED_FROM)
         rm -v -r -f "${targetDirectory}${path}${filename}";
         ;;
      *)
         echo "Unhandled operation: ${operation}";
         ;;
   esac;
done;