8043179: Lambda expression can mutate final field #10381

archiecobbs · 2022-09-21T23:03:45Z

(Description revised)

JDK-8043176 updated the JLS 16.1.10 to describe how within a lambda body, any variables that were DU before the lambda expression must not be considered DU at the start of the lambda block, because arbitrary code could have executed in the meantime:

No rule allows V to be definitely unassigned before a lambda body. This is by design: a variable that was definitely unassigned before the lambda body may end up being assigned to later on, so we cannot conclude that the variable will be unassigned when the body is executed.

This fix to Flow.AssignAnalyzer follows in a straightforward way: mark all variables as not DU before recursing into the lambda body.

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Issue

JDK-8043179: Lambda expression can mutate final field

Reviewers

Vicente Romero (@vicente-romero-oracle - Reviewer) ⚠️ Review applies to 86808b2b

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Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/10381/head:pull/10381
$ git checkout pull/10381

Update a local copy of the PR:
$ git checkout pull/10381
$ git pull https://git.openjdk.org/jdk.git pull/10381/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 10381

View PR using the GUI difftool:
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https://git.openjdk.org/jdk/pull/10381.diff

bridgekeeper · 2022-09-21T23:05:48Z

👋 Welcome back archiecobbs! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

openjdk · 2022-09-21T23:08:49Z

@archiecobbs The following label will be automatically applied to this pull request:

compiler

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mlbridge · 2022-09-21T23:12:51Z

Webrevs

archiecobbs · 2022-09-24T23:23:18Z

I've updated this pull request to include a new unit test (apologies for omitting it previously).

Also there are a couple of changes to other unit tests which get broken because this change reflects changes to the JLS spec per JDK-8043176. I'm a little unclear on precisely what the new behavior is supposed to be, but in any case the actual behavior now makes a lot more sense: basically, everything that happens in a lambda is assumed to happen some indefinite time later in the future (same as if you were instantiating the equivalent local class), so the previous errors expected by T8024809/SelfInitializerInLambdaTestb.java regarding instance field initialization are no longer errors.

In summary:

Added new unit test LambdaMutateFinalField.java testing JDK-8043179 directly.
Unit test lambda/self_initializer/T8024809/SelfInitializerInLambdaTestb.java is now obsolete. Now it compiles successfully.
Unit test lambda/lambdaExpression/EffectivelyFinal_neg.java still works, but no longer generates a duplicate error. I think this is a good thing? The expected test output has been updated.

With the above changes now all javac tests should succeed.

openjdk-notifier · 2022-09-25T00:46:05Z

@archiecobbs Please do not rebase or force-push to an active PR as it invalidates existing review comments. All changes will be squashed into a single commit automatically when integrating. See OpenJDK Developers’ Guide for more information.

…les. Before this change, compiling this class: class LambdaMutateFinalVar { LambdaMutateFinalVar() { final String x; Runnable r1 = () -> x = "not ok"; x = "ok"; } } would report this error: local variables referenced from a lambda expression must be final or effectively final That error is not really appropriate; after all, the variable IS final. The real problem is that it can't be assigned from within the lambda because it can't be assumed to be DU. After this change, this error is reported instead: variable x might already have been assigned

openjdk-notifier · 2022-10-18T17:17:12Z

@archiecobbs Please do not rebase or force-push to an active PR as it invalidates existing review comments. All changes will be squashed into a single commit automatically when integrating. See OpenJDK Developers’ Guide for more information.

bridgekeeper · 2022-11-17T05:53:10Z

@archiecobbs This pull request has been inactive for more than 4 weeks and will be automatically closed if another 4 weeks passes without any activity. To avoid this, simply add a new comment to the pull request. Feel free to ask for assistance if you need help with progressing this pull request towards integration!

bridgekeeper · 2022-12-16T00:00:15Z

@archiecobbs This pull request has been inactive for more than 8 weeks and will now be automatically closed. If you would like to continue working on this pull request in the future, feel free to reopen it! This can be done using the /open pull request command.

archiecobbs · 2022-12-16T00:05:54Z

/open

openjdk · 2022-12-16T00:06:57Z

@archiecobbs This pull request is now open

bridgekeeper · 2023-01-16T20:17:03Z

@archiecobbs This pull request has been inactive for more than 4 weeks and will be automatically closed if another 4 weeks passes without any activity. To avoid this, simply add a new comment to the pull request. Feel free to ask for assistance if you need help with progressing this pull request towards integration!

archiecobbs · 2023-01-19T17:19:28Z

/open

openjdk · 2023-01-19T17:21:01Z

@archiecobbs This pull request is already open

bridgekeeper · 2023-02-16T21:52:08Z

@archiecobbs This pull request has been inactive for more than 4 weeks and will be automatically closed if another 4 weeks passes without any activity. To avoid this, simply add a new comment to the pull request. Feel free to ask for assistance if you need help with progressing this pull request towards integration!

archiecobbs · 2023-02-17T00:02:36Z

/keepalive

openjdk · 2023-02-17T00:03:21Z

@archiecobbs Unknown command keepalive - for a list of valid commands use /help.

bridgekeeper · 2023-03-17T04:13:23Z

@archiecobbs This pull request has been inactive for more than 4 weeks and will be automatically closed if another 4 weeks passes without any activity. To avoid this, simply add a new comment to the pull request. Feel free to ask for assistance if you need help with progressing this pull request towards integration!

archiecobbs · 2023-03-17T13:26:50Z

/pingbot

openjdk · 2023-03-17T13:28:43Z

@archiecobbs Unknown command pingbot - for a list of valid commands use /help.

vicente-romero-oracle · 2023-03-21T04:17:02Z

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Flow.java

+                // body. This is by design: a variable that was definitely unassigned before the
+                // lambda body may end up being assigned to later on, so we cannot conclude that
+                // the variable will be unassigned when the body is executed.
+                uninits.excludeFrom(firstadr);


I think this will exclude all tracked variables not only fields, shouldn't we exclude fields only?

This bug refers only to final fields, but JLS section 16 applies to both fields and variables, so I think this code is following the spec as written.

Here's an example of where this change would catch an illegal final variable assignment:

class LambdaMutateFinalVariable { void foo() { final String x; Runnable r1 = () -> x = "not ok"; x = "ok"; } }

So I don't think the code in the PR is wrong. However, there is a subtlety: currently, the compiler already correctly handles the above example, but that's only by coincidence and thanks to the separate logic for "effectively final":

LambdaMutateFinalVariable.java:4: error: local variables referenced from a lambda expression must be final or effectively final Runnable r1 = () -> x = "not ok"; ^

With the change in this PR, the DA/DU analysis will instead get there first, and so you get a different error:

LambdaMutateFinalVariable.java:4: error: variable x might already have been assigned Runnable r1 = () -> x = "not ok"; ^

It seems to me either error is appropriate, because the code violates both rules. But I think the change in the error message is actually an improvement, because the current error is complaining about the variable not being final even though it actually is final, which is confusing.

Your thoughts?

yep I agree that the error message is better with the patch. My concern was not that the patch was wrong but about the fact that we were applying the change for all variables and fields, but yes it probably makes sense to go for this and have the side effect of better error messages

vicente-romero-oracle

looks good

openjdk · 2023-03-21T14:51:38Z

@archiecobbs This change now passes all automated pre-integration checks.

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After integration, the commit message for the final commit will be:

8043179: Lambda expression can mutate final field

Reviewed-by: vromero

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 34 new commits pushed to the master branch:

bf917ba: 8304687: Move add_to_hierarchy
63d4afb: 8304671: javac regression: Compilation with --release 8 fails on underscore in enum identifiers
e2cfcfb: 6817009: Action.SELECTED_KEY not toggled when using key binding
af4d560: 8303951: Add asserts before record_method_not_compilable where possible
c433862: 6245410: javax.swing.text.html.CSS.Attribute: BACKGROUND_POSITION is not w3c spec compliant
91f407d: 8029301: Confusing error message for array creation method reference
e73411a: 8304376: Rename t1/t2 classes in com/sun/jdi/CLETest.java to avoid class duplication error in IDE
a2d8f63: 8288730: Add type parameter to Lookup::accessClass and Lookup::ensureInitialized
3777455: 8302191: Performance degradation for float/double modulo on Linux
760c012: 8304683: Memory leak in WB_IsMethodCompatible
... and 24 more: https://git.openjdk.org/jdk/compare/42723dcb1862da598092bb499056940d78a8bdac...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

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➡️ To flag this PR as ready for integration with the above commit message, type /integrate in a new comment. (Afterwards, your sponsor types /sponsor in a new comment to perform the integration).

archiecobbs · 2023-03-21T15:14:29Z

/integrate

openjdk · 2023-03-21T15:15:44Z

@archiecobbs
Your change (at version 86808b2) is now ready to be sponsored by a Committer.

vicente-romero-oracle · 2023-03-23T03:19:42Z

/sponsor

openjdk · 2023-03-23T03:20:23Z

@vicente-romero-oracle The PR has been updated since the change author (@archiecobbs) issued the integrate command - the author must perform this command again.

archiecobbs · 2023-03-23T14:08:32Z

/integrate

openjdk · 2023-03-23T14:10:40Z

@archiecobbs
Your change (at version e1cd797) is now ready to be sponsored by a Committer.

vicente-romero-oracle · 2023-03-23T15:58:18Z

/sponsor

openjdk · 2023-03-23T16:00:09Z

Going to push as commit c00d088.
Since your change was applied there have been 35 commits pushed to the master branch:

147f347: 8219083: java/net/MulticastSocket/SetGetNetworkInterfaceTest.java failed in same binary run on windows x64
bf917ba: 8304687: Move add_to_hierarchy
63d4afb: 8304671: javac regression: Compilation with --release 8 fails on underscore in enum identifiers
e2cfcfb: 6817009: Action.SELECTED_KEY not toggled when using key binding
af4d560: 8303951: Add asserts before record_method_not_compilable where possible
c433862: 6245410: javax.swing.text.html.CSS.Attribute: BACKGROUND_POSITION is not w3c spec compliant
91f407d: 8029301: Confusing error message for array creation method reference
e73411a: 8304376: Rename t1/t2 classes in com/sun/jdi/CLETest.java to avoid class duplication error in IDE
a2d8f63: 8288730: Add type parameter to Lookup::accessClass and Lookup::ensureInitialized
3777455: 8302191: Performance degradation for float/double modulo on Linux
... and 25 more: https://git.openjdk.org/jdk/compare/42723dcb1862da598092bb499056940d78a8bdac...master

Your commit was automatically rebased without conflicts.

openjdk · 2023-03-23T16:00:35Z

@vicente-romero-oracle @archiecobbs Pushed as commit c00d088.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

openjdk bot added the rfr Pull request is ready for review label Sep 21, 2022

openjdk bot added the compiler compiler-dev@openjdk.org label Sep 21, 2022

archiecobbs force-pushed the JDK-8043179 branch from df8a09a to 47df9c9 Compare September 24, 2022 23:23

archiecobbs changed the title ~~8043179: Lambda expression can mutate final field.~~ 8043179: Lambda expression can mutate final field Sep 24, 2022

archiecobbs added 6 commits October 18, 2022 12:09

8043179: Lambda expression can mutate final field

989627b

Use /nodynamiccopyright/ for "golden file" test per instructions.

5ee88e7

Move unit test into a more appropriate subdirectory.

db7dd0e

Fix @author in test to match github username per instructions.

faac971

Fix previously incorrect logic and update unit tests.

9699523

archiecobbs force-pushed the JDK-8043179 branch from 9352f98 to 06b39a5 Compare October 18, 2022 17:10

archiecobbs added 2 commits October 19, 2022 10:04

Remove @author tags from tests.

b1d5b3c

Update failed test output line numbers after removing @author tags.

0081d19

Merge branch 'master' into JDK-8043179

1e38c30

bridgekeeper bot closed this Dec 16, 2022

openjdk bot reopened this Dec 16, 2022

Merge branch 'master' into JDK-8043179

86808b2

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Merge branch 'master' into JDK-8043179

e1cd797

openjdk bot removed the sponsor Pull request is ready to be sponsored label Mar 21, 2023

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openjdk bot removed ready Pull request is ready to be integrated rfr Pull request is ready for review sponsor Pull request is ready to be sponsored labels Mar 23, 2023

kevinrushforth mentioned this pull request Apr 22, 2023

8305672: Surprising definite assignment error after JDK-8043179 #13366

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8043179: Lambda expression can mutate final field #10381

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