RFR: 8283225: ClassLoader.c produces incorrect OutOfMemory Exception when length is 0 (aix) [v4]

[Alan Bateman]

On Thu, 17 Mar 2022 16:23:14 GMT, Tyler Steele <duke at openjdk.java.net> wrote:

>> As described in the linked issue, NullClassBytesTest fails due an OutOfMemoryError produced on AIX when the test calls defineClass with a byte array of size of 0. The native implementation of defineClass then calls  malloc with a size of 0. On AIX malloc(0) returns NULL, while on other platforms it return a valid address. When NULL is produced by malloc for this reason, ClassLoader.c incorrectly interprets this as a failure due to a lack of memory.
>> 
>> ~~This PR modifies ClassLoader.c to produce an OutOfMemoryError only when `errno == ENOMEM` and to produce a ClassFormatError with the message "ClassLoader internal allocation failure" in all other cases (in which malloc returns NULL).~~ [edit: The above no longer describes the PR's proposed fix. See discussion below]
>> 
>> In addition, I performed some minor tidy-up work in ClassLoader.c by changing instances of `return 0` to `return NULL`, and `if (some_ptr == 0)` to `if (some_ptr == NULL)`. This was done to improve the clarity of the code in ClassLoader.c, but didn't feel worthy of opening a separate issue.
>> 
>> ### Alternatives
>> 
>> It would be possible to address this failure by modifying the test to accept the OutOfMemoryError on AIX. I thought it was a better solution to modify ClassLoader.c to produce an OutOfMemoryError only when the system is actually out of memory.
>> 
>> ### Testing
>> 
>> This change has been tested on AIX and Linux/x86.
>
> Tyler Steele has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Improve comment
>   
>   - Reword to avoid double use of malloc(X)
>   - Remove bug id

src/java.base/share/native/libjava/ClassLoader.c line 104:

> 102:     // On AIX malloc(0) returns NULL which looks like an out-of-memory condition; so adjust it to malloc(1)
> 103:     #ifdef _AIX
> 104:     body = (jbyte *)malloc(length == 0 ? 1 : length);

Can we use identification in the ifdef/else/endif block to make it a bit more readable. Also can you trim down the comment or split it over two lines to avoid the really long line (it makes it a bit easier for future side-by-side reviews).

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PR: https://git.openjdk.java.net/jdk/pull/7829