[c-ares] fix spin loop bug when c-ares gives up on a socket that still has data left in its read buffer #34185
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| // 4) Because the first two bytes were zero, c-ares attempts to malloc a | ||
| // zero-length buffer: | ||
| // https://github.com/c-ares/c-ares/blob/6360e96b5cf8e5980c887ce58ef727e53d77243a/src/lib/ares_process.c#L428. | ||
| // 5) Because malloc(0) returns NULL, c-ares invokes handle_error and stops |
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Just a small nit: maybe say c-ares' default_malloc(0) returns NULL instead? Since it seems like some systems may return a valid pointer on malloc(0): https://github.com/c-ares/c-ares/blob/main/src/lib/ares_library_init.c#L38-L42
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If we get a readable event on an fd and both the following happens:
c-ares does not read all bytes off the fd
c-ares removes the fd from the set ARES_GETSOCK_READABLE
... then we have a busy loop here, where we'd keep asking c-ares to process an fd that it no longer cares about.
This is indirectly related to a change in this code one month ago: #33942 - before that change, c-ares would close the socket when it called handle_error and so
IsFdStillReadableLockedwould start returningfalse, causing us to get away with this loop. Now, becauseIsFdStillReadableLockedwill keep returning true (because of our overriddencloseAPI), we'll loop forever.The test illustrates one concrete example of how this bug can be hit.
Note that the EE version of this code already gets this right.
Related: internal issue b/297538255