RFR: 8015831: Add lint check for calling overridable methods from a constructor [v7]

Maurizio Cimadamore mcimadamore at openjdk.org
Wed Jan 11 16:02:16 UTC 2023

On Wed, 11 Jan 2023 03:30:03 GMT, Archie L. Cobbs <duke at openjdk.org> wrote:

>> This PR adds a new lint warning category `this-escape`.
>> It also adds `@SuppressWarnings` annotations as needed to the JDK itself to allow the JDK to continue to compile with `-Xlint:all`.
>> A 'this' escape warning is generated for a constructor `A()` in a class `A` when the compiler detects that the following situation is _in theory possible:_
>> * Some subclass `B extends A` exists, and `B` is defined in a separate source file (i.e., compilation unit)
>> * Some constructor `B()` of `B` invokes `A()` as its superclass constructor
>> * During the execution of `A()`, some non-static method of `B.foo()` could get invoked, perhaps indirectly
>> In the above scenario, `B.foo()` would execute before `A()` has returned and before `B()` has performed any initialization. To the extent `B.foo()` accesses any fields of `B` - all of which are still uninitialized - it is likely to function incorrectly.
>> Note, when determining if a 'this' escape is possible, the compiler makes no assumptions about code outside of the current compilation unit. It doesn't look outside of the current source file to see what might actually happen when a method is invoked. It does follow method and constructors within the current compilation unit, and applies a simplified union-of-all-possible-branches data flow analysis to see where 'this' could end up.
>> From my review, virtually all of the warnings generated in the various JDK modules are valid warnings in the sense that a 'this' escape, as defined above, is really and truly possible. However, that doesn't imply that any bugs were found within the JDK - only that the possibility of a certain type of bug exists if certain superclass constructors are used by someone, somewhere, someday.
>> For several "popular" classes, this PR also adds `@implNote`'s to the offending constructors so that subclass implementors are made aware of the threat. For one example, `TreeMap(Map)` invokes `putAll()` and `put()`.
>> More details and a couple of motivating examples are given in an included [doc file](https://github.com/archiecobbs/jdk/blob/ThisEscape/src/java.base/share/classes/java/lang/doc-files/ThisEscape.html) that these `@implNote`'s link to. See also the recent thread on `amber-dev` for some background.
>> Ideally, over time the owners of the various modules would review their `@SuppressWarnings("this-escape")` annotations and determine which other constructors also warranted such an `@implNote`.
>> Because of all the`@SuppressWarnings` annotations, this PR touches a bunch of different JDK modules. My apologies for that. Adding these annotations was determined to be the more conservative approach, as compared to just excepting `this-escape` from various module builds globally.
>> **Patch Navigation Guide**
>> * Non-trivial compiler changes:
>>     * `src/jdk.compiler/share/classes/com/sun/tools/javac/code/Lint.java`
>>     * `src/jdk.compiler/share/classes/com/sun/tools/javac/code/Types.java`
>>     * `src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Flow.java`
>>     * `src/jdk.compiler/share/classes/com/sun/tools/javac/tree/TreeInfo.java`
>>     * `src/jdk.compiler/share/classes/com/sun/tools/javac/comp/ThisEscapeAnalyzer.java`
>>     * `src/jdk.compiler/share/classes/com/sun/tools/javac/resources/compiler.properties`
>>     * `src/jdk.compiler/share/classes/com/sun/tools/javac/resources/javac.properties`
>> * Javadoc additions of `@implNote`:
>>     * `src/java.base/share/classes/java/io/PipedReader.java`
>>     * `src/java.base/share/classes/java/io/PipedWriter.java`
>>     * `src/java.base/share/classes/java/lang/Throwable.java`
>>     * `src/java.base/share/classes/java/util/ArrayDeque.java`
>>     * `src/java.base/share/classes/java/util/EnumMap.java`
>>     * `src/java.base/share/classes/java/util/HashSet.java`
>>     * `src/java.base/share/classes/java/util/Hashtable.java`
>>     * `src/java.base/share/classes/java/util/LinkedList.java`
>>     * `src/java.base/share/classes/java/util/TreeMap.java`
>>     * `src/java.base/share/classes/java/util/TreeSet.java`
>> * New unit tests
>>     * `test/langtools/tools/javac/warnings/ThisEscape/*.java`
>> * **Everything else** is just adding `@SuppressWarnings("this-escape")`
> Archie L. Cobbs has updated the pull request incrementally with two additional commits since the last revision:
>  - Use the more appropriate Type comparison method Types.isSameType().
>  - Add some more comments to clarify how the analysis works.

More questions. Let's say that we only tracked escaping of direct references to `this`. Do you have any sense of how many "less" warnings would be reported in real code (e.g. JDK) ? What I mean by this - let's say that we don't care about tracking _where_ `this` ends up going exactly (e.g. if it's aliased by another variable) - and perhaps also let's say we don't care about inspecting the method to which `this` is leaked too closely - e.g. we treat any early escape of `this` as a possible issue. Of course you could construct examples (like your tests) in which such a simplistic analysis would be defeated. What I'm interested in though is what incremental improvement is brought by the more complex analysis you have in this PR?


PR: https://git.openjdk.org/jdk/pull/11874

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