Would you like to own the deprecation and removal of the feature?

I think we should act sooner rather than later, to make sure this doesn't become used. Hence making it high priority. If you don't want the bug, I can take it or find someone who does.

If you want to get usage statistics,  issue 376039  tracks adding use counter support from service workers.

Yes, UseCounter for Service Worker will be available maybe soon (in Q1?). However, UseCounter doesn't record what kind of GlobalScope uses the feature, so it wouldn't be so useful in this case where we want to know usage only on ServiceWorkerGlobalScope...

(+rbyers@ to CC for an interesting (rare?) use case of UseCounter)

Off on a tangent, but I just got a ping from Microsoft wondering about collecting stats on a particular Indexed DB case, distinguishing SW from non-SW use.

We could add a UseCounter::Feature only for ServiceWorker like RequestFileSystemWorker. This way is not scalable but feasible after SW UseCounter is enabled.

Yeah, having use counters split out by global type might be nice to have in general, but for individual cases it's not that hard to add custom use counters for specific global types. So I guess the question here is where the balance between waiting longer to delete this so we can collect data on usage, vs just deleting it sooner to avoid more usage should be.

IMHO we should deprecate FileReaderSync immediately, to mitigate the risk of adoption, and remove as soon as we can.

Either way is fine with me. I'm assuming the rate of FileReaderSync use on ServiceWorker is quite low.

Yeah, I'm assuming the same. Seems two sensible options would be:
1) remove completely in M58, hoping nothing breaks, or
2) deprecate in M58, hopefully usecounters will also land in M58, and then unless usage turns out to be too high remove in M59

How about deprecating in M58, and attempting to merge the deprecation to M57?

Merging the deprecation warning to M57 might make sense, yes. I don't think there is any hope to get use counters for service workers merged to M57 as well though, but just having the warning would still be nice of course.

I started drafting an intent-to-deprecate at https://docs.google.com/document/d/1xHi38EhijVnb8HCbk1MN1MLbeGnKq6gfd5PWSk6MvHo/edit, comments welcome

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/77f740e334ba6fdb4d6493372e8009c544939ed4

commit 77f740e334ba6fdb4d6493372e8009c544939ed4
Author: mek <mek@chromium.org>
Date: Sat Feb 11 02:20:29 2017

Add a use counter and histogram to FileReaderSync.

To hopefully eventually facilitate removing FileReaderSync from service
workers this adds both a use counter and histogram to log how often the
API is used in the various worker types.

BUG= 688586 

Review-Url: https://codereview.chromium.org/2682223006
Cr-Commit-Position: refs/heads/master@{#449833}

[modify] https://crrev.com/77f740e334ba6fdb4d6493372e8009c544939ed4/third_party/WebKit/Source/core/fileapi/FileReaderSync.cpp
[modify] https://crrev.com/77f740e334ba6fdb4d6493372e8009c544939ed4/third_party/WebKit/Source/core/fileapi/FileReaderSync.h
[modify] https://crrev.com/77f740e334ba6fdb4d6493372e8009c544939ed4/third_party/WebKit/Source/core/fileapi/FileReaderSync.idl
[modify] https://crrev.com/77f740e334ba6fdb4d6493372e8009c544939ed4/third_party/WebKit/Source/core/frame/UseCounter.h
[modify] https://crrev.com/77f740e334ba6fdb4d6493372e8009c544939ed4/tools/metrics/histograms/histograms.xml

I'd like to merge the UMA change from comment #13 to M57 in order to collect data sooner.

Your change meets the bar and is auto-approved for M57. Please go ahead and merge the CL to branch 2987 manually. Please contact milestone owner if you have questions.
Owners: amineer@(clank), cmasso@(bling), ketakid@(cros), govind@(desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f9a5a3f987c9c5c4af122effef4440d97838e568

commit f9a5a3f987c9c5c4af122effef4440d97838e568
Author: Marijn Kruisselbrink <mek@chromium.org>
Date: Mon Feb 13 21:57:39 2017

Add a use counter and histogram to FileReaderSync.

To hopefully eventually facilitate removing FileReaderSync from service
workers this adds both a use counter and histogram to log how often the
API is used in the various worker types.

BUG= 688586 

Review-Url: https://codereview.chromium.org/2682223006
Cr-Commit-Position: refs/heads/master@{#449833}
(cherry picked from commit 77f740e334ba6fdb4d6493372e8009c544939ed4)

Review-Url: https://codereview.chromium.org/2694093002 .
Cr-Commit-Position: refs/branch-heads/2987@{#489}
Cr-Branched-From: ad51088c0e8776e8dcd963dbe752c4035ba6dab6-refs/heads/master@{#444943}

[modify] https://crrev.com/f9a5a3f987c9c5c4af122effef4440d97838e568/third_party/WebKit/Source/core/fileapi/FileReaderSync.cpp
[modify] https://crrev.com/f9a5a3f987c9c5c4af122effef4440d97838e568/third_party/WebKit/Source/core/fileapi/FileReaderSync.h
[modify] https://crrev.com/f9a5a3f987c9c5c4af122effef4440d97838e568/third_party/WebKit/Source/core/fileapi/FileReaderSync.idl
[modify] https://crrev.com/f9a5a3f987c9c5c4af122effef4440d97838e568/third_party/WebKit/Source/core/frame/UseCounter.h
[modify] https://crrev.com/f9a5a3f987c9c5c4af122effef4440d97838e568/tools/metrics/histograms/histograms.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d039981e6de2c11e1dfdc1c0f6ba61804cc92c49

commit d039981e6de2c11e1dfdc1c0f6ba61804cc92c49
Author: mek <mek@chromium.org>
Date: Wed Feb 15 06:28:09 2017

Deprecate FileReaderSync in service workers.

Intent to deprecate and remove: https://groups.google.com/a/chromium.org/d/msg/blink-dev/cjWtqRD6iw8/hcxtAeE2CAAJ

BUG= 688586 

Review-Url: https://codereview.chromium.org/2680363003
Cr-Commit-Position: refs/heads/master@{#450593}

[modify] https://crrev.com/d039981e6de2c11e1dfdc1c0f6ba61804cc92c49/third_party/WebKit/Source/core/fileapi/FileReaderSync.cpp
[modify] https://crrev.com/d039981e6de2c11e1dfdc1c0f6ba61804cc92c49/third_party/WebKit/Source/core/frame/Deprecation.cpp
[modify] https://crrev.com/d039981e6de2c11e1dfdc1c0f6ba61804cc92c49/third_party/WebKit/Source/core/frame/UseCounter.h
[modify] https://crrev.com/d039981e6de2c11e1dfdc1c0f6ba61804cc92c49/tools/metrics/histograms/histograms.xml

I'd also like to backport the change in commend #17 to M57

Your change meets the bar and is auto-approved for M57. Please go ahead and merge the CL to branch 2987 manually. Please contact milestone owner if you have questions.
Owners: amineer@(clank), cmasso@(bling), ketakid@(cros), govind@(desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/389c6611afa3e464d8e6c7fb2eda6f02d6e8cdc9

commit 389c6611afa3e464d8e6c7fb2eda6f02d6e8cdc9
Author: Marijn Kruisselbrink <mek@chromium.org>
Date: Thu Feb 16 19:55:29 2017

Deprecate FileReaderSync in service workers.

Intent to deprecate and remove: https://groups.google.com/a/chromium.org/d/msg/blink-dev/cjWtqRD6iw8/hcxtAeE2CAAJ

BUG= 688586 

Review-Url: https://codereview.chromium.org/2680363003
Cr-Commit-Position: refs/heads/master@{#450593}
(cherry picked from commit d039981e6de2c11e1dfdc1c0f6ba61804cc92c49)

Review-Url: https://codereview.chromium.org/2704573002 .
Cr-Commit-Position: refs/branch-heads/2987@{#555}
Cr-Branched-From: ad51088c0e8776e8dcd963dbe752c4035ba6dab6-refs/heads/master@{#444943}

[modify] https://crrev.com/389c6611afa3e464d8e6c7fb2eda6f02d6e8cdc9/third_party/WebKit/Source/core/fileapi/FileReaderSync.cpp
[modify] https://crrev.com/389c6611afa3e464d8e6c7fb2eda6f02d6e8cdc9/third_party/WebKit/Source/core/frame/Deprecation.cpp
[modify] https://crrev.com/389c6611afa3e464d8e6c7fb2eda6f02d6e8cdc9/third_party/WebKit/Source/core/frame/UseCounter.h
[modify] https://crrev.com/389c6611afa3e464d8e6c7fb2eda6f02d6e8cdc9/tools/metrics/histograms/histograms.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2a0319f1e13c5d4f7e8eb7771a8818f31a951b01

commit 2a0319f1e13c5d4f7e8eb7771a8818f31a951b01
Author: mek <mek@chromium.org>
Date: Tue Mar 21 18:40:00 2017

Disable FileReaderSync in service workers.

Usage so far in service workers is zero, so go ahead with disabling
FileReaderSync in service workers as planned.

Intent to deprecate and remove:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/cjWtqRD6iw8/hcxtAeE2CAAJ

BUG= 688586 

Review-Url: https://codereview.chromium.org/2753943002
Cr-Commit-Position: refs/heads/master@{#458487}

[modify] https://crrev.com/2a0319f1e13c5d4f7e8eb7771a8818f31a951b01/third_party/WebKit/LayoutTests/external/wpt/FileAPI/historical.https-expected.txt
[modify] https://crrev.com/2a0319f1e13c5d4f7e8eb7771a8818f31a951b01/third_party/WebKit/LayoutTests/http/tests/serviceworker/webexposed/global-interface-listing-service-worker-expected.txt
[modify] https://crrev.com/2a0319f1e13c5d4f7e8eb7771a8818f31a951b01/third_party/WebKit/LayoutTests/virtual/service-worker-navigation-preload/http/tests/serviceworker/webexposed/global-interface-listing-service-worker-expected.txt
[modify] https://crrev.com/2a0319f1e13c5d4f7e8eb7771a8818f31a951b01/third_party/WebKit/LayoutTests/virtual/stable/http/tests/serviceworker/webexposed/global-interface-listing-service-worker-expected.txt
[modify] https://crrev.com/2a0319f1e13c5d4f7e8eb7771a8818f31a951b01/third_party/WebKit/Source/core/fileapi/FileReaderSync.cpp
[modify] https://crrev.com/2a0319f1e13c5d4f7e8eb7771a8818f31a951b01/third_party/WebKit/Source/core/fileapi/FileReaderSync.idl
[modify] https://crrev.com/2a0319f1e13c5d4f7e8eb7771a8818f31a951b01/third_party/WebKit/Source/core/frame/Deprecation.cpp
[modify] https://crrev.com/2a0319f1e13c5d4f7e8eb7771a8818f31a951b01/third_party/WebKit/Source/core/frame/UseCounter.h