[sanerdemirel]

Fixes #9342.

UpdaterClass::begin() picks a 4 KB write buffer when total free heap is above 8 KB, and a 256 byte buffer otherwise. It then makes one allocation. If the heap is fragmented, there can be more than 8 KB free but no free block of 4 KB. The allocation fails and begin() returns UPDATE_ERROR_OOM, although the update would work with 256 bytes.

As suggested in the issue, begin() now walks through {FLASH_SECTOR_SIZE, 256} and uses new (std::nothrow) for each size. The first size that works is used. If both fail, it still reports UPDATE_ERROR_OOM.

The low-heap rule is unchanged: with 8 KB or less free, only 256 is tried, so the buffer does not take the last of the heap.

The diff is cores/esp8266/Updater.cpp only, +7/-5. The 256 byte path was already used when the heap was low, so no code needs to learn about a new buffer size.

Tests

This PR adds no new tests. An earlier version had host tests that replaced the nothrow operator new[] for the whole test binary, to refuse chosen sizes. As discussed in the review, that is the wrong level, and under valgrind the hook is never called, so I dropped them. The fallback tests can follow on top of a mocking allocator that works under valgrind:

• 4096 refused: begin() still succeeds with 256.
• 4096 and 256 refused: begin() reports UPDATE_ERROR_OOM, and the updater can be used again afterwards.

Results on the current head, on Ubuntu 24.04 with g++ 13 and clang-format-18:

• make FORCE32=0 D=1 test passes: 89 test cases, 33214 assertions. The existing Updater host tests run against the changed begin().
• make CI (-Werror, valgrind, coverage) passes: 0 errors, no leaks reported.
• tests/restyle.sh with clang-format-18 changes nothing.

Not tested

• The fallback path itself, by a test (see above).
• I did not run this on hardware. The change is only the buffer choice in begin().
• I did not measure upload speed with the 256 byte buffer. That path was already in use for low heap.
• Only g++ 13 on x86-64 (64-bit mode, as in CI) was used. I did not build the core for the ESP8266.

[sanerdemirel]

Thanks for the review. You are right: replacing operator new[] for the whole test runtime is the wrong level, and under valgrind the hook is never called, so those tests only skipped themselves there.

I have dropped the allocation-hook tests from this PR. What is left is the change in Updater.cpp itself (try FLASH_SECTOR_SIZE, then 256 B, under the same low-heap rule as before). The existing Updater host tests still pass against it, also under make CI with valgrind.

When your mocking allocator with -Wl,--wrap is in, I am happy to add the fallback tests on top of it: one that refuses the 4 KB buffer and checks that begin() still succeeds with 256 B, and one that refuses both and checks the clean out-of-memory error.