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flags: flag expiration support, take 2
This change implements flag expiration, with the same goal but a very different method than the previous implementation (https://chromium-review.googlesource.com/c/chromium/src/+/1729449). This change preserves the flag-metadata.json file as the sole authoritative source for flag expiration data. During the build process, a new script (//tools/flags/generate_expired_list.py) generates a data structure from flag-metadata.json listing flags that are expired at or before the current milestone, with their expiration milestones. Logic in //chrome/browser/about_flags.cc and //chrome/browser/unexpire_flags.cc then conditionally hides these flags, depending on the state of the unexpire flags - see //doc/flag_expiry.md for details about those. Specifically, this change: 1) Renames //tools/flags/list-flags.py to list_flags.py to make it importable as a module; 2) Introduces //chrome/browser/expired_flags_list.h, which describes a data structure for representing expired flags; 3) Introduces //tools/flags/generate_expired_list.py, which takes as inputs //chrome/browser/flag-metadata.json and //chrome/VERSION and generates a C++ source file containing a data structure matching that in expired_flags_list.h; 4) Has the //chrome/browser build target depend on the generated file from step 3; 5) Re-introduces an unexpire flag as described in //doc/flag_expiry.md; 6) Adds logic to //chrome/browser/about_flags.cc to conditionally hide expired flags. This means that flag-metadata.json is the sole determinant of whether a flag is currently expired or not. Note that this change expires all flags whose expiration milestones were *M76* or earlier, not M78 (the current mstone); this is because the backlog of flags to expire is currently quite large. In M79, we will expire flags <= M78, then in M80, we will expire flags <= M80; after that, each milestone Mx will expire flags <= Mx. Also note that this change discards the notion of flag expiry depending on usage metrics - i.e., we will now expire *all* flags that expire before the target milestone, not just the N with the lowest usage. The lowest-usage design proved to be extremely manual to implement and very difficult for developers to predict the results of. Bug: 953690 Change-Id: I7389b667d1e032c795c137cafadf0e5e80fb82c1 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1754762 Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org> Reviewed-by: Nico Weber <thakis@chromium.org> Cr-Commit-Position: refs/heads/master@{#690742}
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,24 @@ | ||
| // Copyright 2019 The Chromium Authors. All rights reserved. | ||
| // Use of this source code is governed by a BSD-style license that can be | ||
| // found in the LICENSE file. | ||
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| #ifndef CHROME_BROWSER_EXPIRED_FLAGS_LIST_H_ | ||
| #define CHROME_BROWSER_EXPIRED_FLAGS_LIST_H_ | ||
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| // This header file declares a data structure that is generated at compile time | ||
| // by //tools/flags/generate_expired_list.py - also see the | ||
| // //chrome/browser:expired_flags_list target. | ||
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| namespace flags { | ||
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| struct ExpiredFlag { | ||
| const char* name; | ||
| int mstone; | ||
| }; | ||
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| // This array of names is terminated with a flag whose name is nullptr. | ||
| extern const ExpiredFlag kExpiredFlags[]; | ||
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| } // namespace flags | ||
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| #endif // CHROME_BROWSER_EXPIRED_FLAGS_LIST_H_ |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,46 @@ | ||
| // Copyright 2019 The Chromium Authors. All rights reserved. | ||
| // Use of this source code is governed by a BSD-style license that can be | ||
| // found in the LICENSE file. | ||
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| #include "chrome/browser/unexpire_flags.h" | ||
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| #include "chrome/browser/expired_flags_list.h" | ||
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| namespace flags { | ||
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| const base::Feature kUnexpireFlagsM76{"TemporaryUnexpireFlagsM76", | ||
| base::FEATURE_DISABLED_BY_DEFAULT}; | ||
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| bool ExpiryEnabledForMstone(int mstone) { | ||
| // generate_expired_list.py will never emit flags with expiry milestone -1, to | ||
| // keep binary size down. However, if a bug *did* cause that to happen, and | ||
| // this function did not handle that case, disaster could ensue: all the -1 | ||
| // flags that are supposed to never expire would in fact expire instantly, | ||
| // since -1 < x for any valid mstone x. | ||
| // As such, there's an extra error-check here: never allow flags with mstone | ||
| // -1 to expire. | ||
| DCHECK(mstone != -1); | ||
| if (mstone == -1) | ||
| return false; | ||
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| // Currently expiration targets flags expiring in M76 or earlier. In M79 this | ||
| // will become M78 or earlier; in M80 it will become M80 or earlier, and in | ||
| // all future milestones Mx it will be Mx or earlier, so this logic will cease | ||
| // to hardcode a milestone and instead target the current major version. | ||
| if (mstone < 76) | ||
| return true; | ||
| if (mstone == 76) | ||
| return !base::FeatureList::IsEnabled(kUnexpireFlagsM76); | ||
| return false; | ||
| } | ||
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| bool IsFlagExpired(const char* internal_name) { | ||
| for (int i = 0; kExpiredFlags[i].name; ++i) { | ||
| const ExpiredFlag* f = &kExpiredFlags[i]; | ||
| if (!strcmp(f->name, internal_name) && ExpiryEnabledForMstone(f->mstone)) | ||
| return true; | ||
| } | ||
| return false; | ||
| } | ||
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| } // namespace flags |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,18 @@ | ||
| // Copyright 2019 The Chromium Authors. All rights reserved. | ||
| // Use of this source code is governed by a BSD-style license that can be | ||
| // found in the LICENSE file. | ||
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| #ifndef CHROME_BROWSER_UNEXPIRE_FLAGS_H_ | ||
| #define CHROME_BROWSER_UNEXPIRE_FLAGS_H_ | ||
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| #include "base/feature_list.h" | ||
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| namespace flags { | ||
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| extern const base::Feature kUnexpireFlagsM76; | ||
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| bool IsFlagExpired(const char* internal_name); | ||
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| } // namespace flags | ||
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| #endif // CHROME_BROWSER_UNEXPIRE_FLAGS_H_ |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,125 @@ | ||
| #!/usr/bin/env python | ||
| # Copyright 2019 The Chromium Authors. All rights reserved. | ||
| # Use of this source code is governed by a BSD-style license that can be | ||
| # found in the LICENSE file. | ||
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| """Generates the list of expired flags as a C++ source file. | ||
| This program generates a data structure representing the set of flags that | ||
| expired before or as of the current Chromium milestone. Specifically, it reads | ||
| the flag metadata JSON file and emits a structure mapping flag internal names to | ||
| expiration milestones. This data structure is then linked into the built | ||
| Chromium, to be used to decide whether to show or hide a given flag in the flags | ||
| UI. | ||
| This program can be run with no arguments to run its own unit tests. | ||
| """ | ||
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| import list_flags | ||
| import os | ||
| import sys | ||
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| ROOT_PATH = os.path.join(os.path.dirname(__file__), '..', '..') | ||
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| def get_chromium_version(): | ||
| """Parses the Chromium version out of //chrome/VERSION.""" | ||
| with open(os.path.join(ROOT_PATH, 'chrome', 'VERSION')) as f: | ||
| for line in f.readlines(): | ||
| key, value = line.strip().split('=') | ||
| if key == 'MAJOR': | ||
| return value | ||
| return None | ||
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| def gen_file_header(prog_name, meta_name): | ||
| """Returns the header for the generated expiry list file. | ||
| The generated header contains at least: | ||
| * A copyright message on the first line | ||
| * A reference to this program (prog_name) | ||
| * A reference to the input metadata file | ||
| >>> 'The Chromium Authors' in gen_file_header('foo', 'bar') | ||
| True | ||
| >>> '/progname' in gen_file_header('/progname', '/dataname') | ||
| True | ||
| >>> '/dataname' in gen_file_header('/progname', '/dataname') | ||
| True | ||
| """ | ||
| return """// Copyright 2019 The Chromium Authors. All rights reserved. | ||
| // Use of this source code is governed by a BSD-style license that can be | ||
| // found in the LICENSE file. | ||
| // This is a generated file. Do not edit it! It was generated by: | ||
| // {prog_name} | ||
| // and it was generated from: | ||
| // {meta_name} | ||
| #include "chrome/browser/expired_flags_list.h" | ||
| namespace flags {{ | ||
| const ExpiredFlag kExpiredFlags[] = {{ | ||
| """.format(prog_name=prog_name, meta_name=meta_name) | ||
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| def gen_file_footer(): | ||
| return """ | ||
| {nullptr, 0}, | ||
| }; | ||
| } // namespace flags""" | ||
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| def gen_file_body(flags, mstone): | ||
| """Generates the body of the flag expiration list. | ||
| Flags appear in the list only if: | ||
| * Their expiration mstone is not -1 | ||
| * Either the chrome version is unknown OR | ||
| * Their expiration mstone is <= the chrome version | ||
| >>> flags = [ { 'name': 'foo', 'expiry_milestone': 1 } ] | ||
| >>> flags.append({ 'name': 'bar', 'expiry_milestone': 2 }) | ||
| >>> flags.append({ 'name': 'baz', 'expiry_milestone': -1 }) | ||
| >>> gen_file_body(flags, 1) | ||
| ' {"foo", 1},' | ||
| >>> gen_file_body(flags, 2) | ||
| ' {"foo", 1},\\n {"bar", 2},' | ||
| >>> gen_file_body(flags, None) | ||
| ' {"foo", 1},\\n {"bar", 2},' | ||
| """ | ||
| if mstone != None: | ||
| flags = list_flags.keep_expired_by(flags, mstone) | ||
| output = [] | ||
| for f in flags: | ||
| if f['expiry_milestone'] != -1: | ||
| name, expiry = f['name'], f['expiry_milestone'] | ||
| output.append(' {"' + name + '", ' + str(expiry) + '},') | ||
| return '\n'.join(output) | ||
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| def gen_expiry_file(program_name, metadata_name): | ||
| output = gen_file_header(program_name, metadata_name) | ||
| output += gen_file_body(list_flags.load_metadata(), get_chromium_version()) | ||
| output += gen_file_footer() | ||
| return output | ||
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| def main(): | ||
| import doctest | ||
| doctest.testmod() | ||
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| if len(sys.argv) < 3: | ||
| print '{}: only ran tests'.format(sys.argv[0]) | ||
| return | ||
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| output = gen_expiry_file(sys.argv[0], sys.argv[1]) | ||
| with open(sys.argv[2], 'w') as f: | ||
| f.write(output) | ||
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| if __name__ == '__main__': | ||
| main() |
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