fix(watch): Ensure that watch tasks are throttled to run one at a time per watcher.

[jthoms1]

This PR should prevent the memory overruns that we have seen from rollup. The code ensures that we are running only watch task at a time by using the returned promises. It also uses a single entry queue so that we only run one additional watch task on previous task completion.

[Manduro]

@jthoms1 Thank you for this fix! Is it possible that saving a lot of files right after each other creates a huge queue for compiling?

Another solution would be to cancel the current task and start a new one right away, instead of waiting for the current task to finish. That should be faster, right?

[jthoms1]

Yes this creates a big queue it seems. This fix should resolve that. I would prefer we let the current finish because I don't want to make any assumptions about the current state at any point in time. Just being safe about it.

[Aristona]

Thanks alot!

[Manduro]

@jthoms1 Just had another one with the latest ionic-app-scripts that included this fix:

[11:59:14]  typescript compilation started ...
[11:59:15]  templateUpdate started ...
[11:59:15]  bundle dev update started ...
[11:59:24]  typescript compilation finished in 10.03 s
[11:59:24]  bundle dev update started ...
[11:59:24]  RangeError: Maximum call stack size exceeded
    at deepClone (/Users/etc/etc/etc/node_modules/rollup/dist/rollup.js:153:20)
    at deepClone (/Users/etc/etc/etc/node_modules/rollup/dist/rollup.js:165:18)
    at deepClone (/Users/etc/etc/etc/node_modules/rollup/dist/rollup.js:165:18)
    at deepClone (/Users/etc/etc/etc/node_modules/rollup/dist/rollup.js:165:18)
    at deepClone (/Users/etc/etc/etc/node_modules/rollup/dist/rollup.js:165:18)
    at deepClone (/Users/etc/etc/etc/node_modules/rollup/dist/rollup.js:165:18)
    at deepClone (/Users/etc/etc/etc/node_modules/rollup/dist/rollup.js:165:18)
    at deepClone (/Users/etc/etc/etc/node_modules/rollup/dist/rollup.js:165:18)
    at deepClone (/Users/etc/etc/etc/node_modules/rollup/dist/rollup.js:165:18)
    at deepClone (/Users/etc/etc/etc/node_modules/rollup/dist/rollup.js:165:18)
[11:59:24]  sass started ...
[11:59:24]  sass finished in 39 ms
JS changed:   www/build/main.js.map
JS changed:   www/build/main.js
[11:59:35]  bundle dev update finished in 20.73 s
[11:59:35]  templateUpdate finished in 20.74 s

[maxkunitsa]

@jthoms1 also got 'Maximum call stack size exceeded' even with version 0.0.27

[ecureuill]

I think it will be necessary to cancel some tasks, maybe conditionally to queue length.
I used to save every line modification and I think this is causing watch to break

0 info it worked if it ends with ok
1 verbose cli [ '/usr/bin/nodejs', '/usr/bin/npm', 'run', 'watch' ]
2 info using [email protected]
3 info using [email protected]
4 verbose run-script [ 'prewatch', 'watch', 'postwatch' ]
5 info lifecycle ionic-hello-world@~prewatch: ionic-hello-world@
6 silly lifecycle ionic-hello-world@~prewatch: no script for prewatch, continuing
7 info lifecycle ionic-hello-world@~watch: ionic-hello-world@
8 verbose lifecycle ionic-hello-world@~watch: unsafe-perm in lifecycle true
9 verbose lifecycle ionic-hello-world@~watch: PATH: /usr/lib/node_modules/npm/bin/node-gyp-bin:/home/sciuro/Projects/node_modules/.bin:/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games:/opt/android-sdk/tools:/opt/android-sdk/platforms-tools
10 verbose lifecycle ionic-hello-world@~watch: CWD: /home/sciuro/Projects
11 silly lifecycle ionic-hello-world@~watch: Args: [ '-c', 'ionic-app-scripts watch' ]
12 silly lifecycle ionic-hello-world@~watch: Returned: code: 134  signal: null
13 info lifecycle ionic-hello-world@~watch: Failed to exec watch script
14 verbose stack Error: ionic-hello-world@ watch: `ionic-app-scripts watch`
14 verbose stack Exit status 134
14 verbose stack     at EventEmitter.<anonymous> (/usr/lib/node_modules/npm/lib/utils/lifecycle.js:255:16)
14 verbose stack     at emitTwo (events.js:87:13)
14 verbose stack     at EventEmitter.emit (events.js:172:7)
14 verbose stack     at ChildProcess.<anonymous> (/usr/lib/node_modules/npm/lib/utils/spawn.js:40:14)
14 verbose stack     at emitTwo (events.js:87:13)
14 verbose stack     at ChildProcess.emit (events.js:172:7)
14 verbose stack     at maybeClose (internal/child_process.js:829:16)
14 verbose stack     at Process.ChildProcess._handle.onexit (internal/child_process.js:211:5)
15 verbose pkgid ionic-hello-world@
16 verbose cwd /home/sciuro/Projects
17 error Linux 3.16.0-4-amd64
18 error argv "/usr/bin/nodejs" "/usr/bin/npm" "run" "watch"
19 error node v4.5.0
20 error npm  v3.10.6
21 error code ELIFECYCLE
22 error ionic-hello-world@ watch: `ionic-app-scripts watch`
22 error Exit status 134
23 error Failed at the ionic-hello-world@ watch script 'ionic-app-scripts watch'.
23 error Make sure you have the latest version of node.js and npm installed.
23 error If you do, this is most likely a problem with the ionic-hello-world package,
23 error not with npm itself.
23 error Tell the author that this fails on your system:
23 error     ionic-app-scripts watch
23 error You can get information on how to open an issue for this project with:
23 error     npm bugs ionic-hello-world
23 error Or if that isn't available, you can get their info via:
23 error     npm owner ls ionic-hello-world
23 error There is likely additional logging output above.
24 verbose exit [ 1, true ]

At least, compared to 0.0.23 version, my memory consumption is better.
I started a new project with 0.0.23 and it was impossible to work on it because of watch.