Add a direction change barrier to the littlefs stream driver - #3527
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Open() opens an existing file with "r+", and Read() and Write() then call fread() and fwrite() on that stream with nothing in between. ANSI C requires a file positioning call or an fflush() between a write and a following read on an update mode stream, and a positioning call between a read and a following write. Without one the behaviour is undefined, and newlib in practice serves the second operation from the buffer state left by the first: a read after a write returns stale bytes, and a write after a read lands at the buffer position instead of the logical file position. Record the direction of the last operation on the handle and seek to the current position when it changes. Positioning calls clear the recorded direction, since they are barriers themselves.
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📝 SummarySummary by CodeRabbit
WalkthroughThe ESP32 LITTLEFS driver now tracks the last operation on each open file. It applies ChangesLittleFS direction barriers
Estimated code review effort: 2 (Simple) | ~10 minutes Merge Risk: 🟡 Moderate · up to Mixed read/write operations on LITTLEFS update streams can proceed after a failed positioning barrier, risking incorrect file reads or writes during storage errors. Handle the barrier failure before merge. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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josesimoes
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josesimoes
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I understand where this is going. No magic numbers here. Please add an enum with the op codes and comment each one along with the enum purpose.
Remove the explanation from the LITTLEFS_FileHandle struct and from the various set/IF checks.
The explanations on the PR are more than enough.
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Inline comments:
In `@targets/ESP32/_littlefs/littlefs_FS_Driver.cpp`:
- Around line 291-293: In both Read and Write, check the return value of the
direction-change fseek(..., SEEK_CUR) barrier and return CLR_E_FILE_IO
immediately on failure, before updating fileHandle->lastOp or performing the
opposite operation. Apply the same handling at the affected sites in
targets/ESP32/_littlefs/littlefs_FS_Driver.cpp:291-293 and
targets/ESP32/_littlefs/littlefs_FS_Driver.cpp:342-344.
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📒 Files selected for processing (2)
targets/ESP32/_littlefs/littlefs_FS_Driver.cpptargets/ESP32/_littlefs/littlefs_FS_Driver.h
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Done in 8031b31. Added |
josesimoes
added
the
Platform: ESP32
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Also took the CodeRabbit point in 0135ef8: a failing |
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/azp run |
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Azure Pipelines successfully started running 2 pipeline(s). |
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The two red ESP32 jobs (build 65642) failed in |
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Description
LITTLEFS_FileHandlerecords the direction of the last operation on the handle.Read()andWrite()issuefseek(file, 0, SEEK_CUR)when the direction changes.Seek(), and the position restore inGetLength()andSetLength(), clear the recorded direction, since a positioning call is a barrier in itself.Motivation and Context
Open()opens an existing file with"r+", andRead()andWrite()then callfread()andfwrite()on that stream with nothing in between. ANSI C requires a file positioning call or anfflush()between a write and a following read on an update mode stream, and a positioning call between a read and a following write. Without one the behaviour is undefined, and newlib in practice serves the second operation from the buffer state left by the first: a read after a write returns stale bytes, and a write after a read lands at the buffer position instead of the logical file position.Any managed code that reads and writes through the same
FileStreamruns into this, a read-modify-write over an existing file being the obvious case.How Has This Been Tested?
main.Types of changes
Checklist