fs/vfs: Report POLLERR in the read and write sets of select().

[casaroli]

depends-on: [/pull/20661 apache/nuttx-apps/pull/3845]

Summary

select() put a descriptor with POLLERR only in exceptfds. A caller that asked only for readfds or writefds, which is the usual case, did not see the error: select() returned 0, also with no timeout, where 0 means that the timeout expired.

POSIX says that a descriptor is ready for reading (or writing) when the call would not block, "whether or not the function would transfer data successfully", so also when it would return an error. Linux reports POLLERR in the read and write sets.

This PR reports POLLERR in readfds and writefds. poll() reports POLLHUP and POLLERR also when they were not asked for, so the PR sets only the bits of the sets that the caller asked for: before, a descriptor in writefds only could also appear in readfds on POLLHUP. exceptfds does not change.

Impact

select() callers that ask for readfds or writefds now see a descriptor with an error as ready, and the following read() or write() returns the error. exceptfds is as before.

Testing

qemu-armv8a:nsh (flat) with CONFIG_PIPES=y and the default CONFIG_DEV_PIPE_NPOLLWAITERS=4. A test program starts 6 threads that call select() with no timeout on the read end of one pipe, then writes 6 bytes. The pipe has 4 poll slots, so threads 5 and 6 get POLLERR:

	master	this PR
select() calls with no timeout that returned 0	2 of 6	0 of 6

With this PR the two threads see the descriptor in readfds, and their read() waits for the data. ostest exits with status 0 on both. Build: qemu-armv8a:knsh and lm3s6965-ek:qemu-flat, no warnings. tools/checkpatch.sh -c -u -m -g passes.

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MemBrowse Memory Report

arduino-mega2560

• flash: .data +16 B (+0.0%, 65,952 B / 262,144 B, total: 25% used)
• sram: .data +16 B (+0.6%, 2,718 B / 8,192 B, total: 33% used)

esp32-devkitc

• ROM: .dram0.data +16 B, .flash.rodata +12 B, .flash.text +60 B (+0.1%, 125,240 B / 4,194,272 B, total: 3% used)
• dram0_0_seg: .dram0.data +16 B (+0.2%, 10,012 B / 180,736 B, total: 6% used)
• drom0_0_seg: .flash.rodata +12 B (+0.1%, 13,472 B / 4,194,272 B, total: 0% used)
• irom0_0_seg: .flash.text +60 B (+0.1%, 89,416 B / 3,342,304 B, total: 3% used)

hifive1-revb

• flash: .data +8 B, .text +88 B (+0.1%, 84,348 B / 4,194,304 B, total: 2% used)
• sram: .bss -16 B, .data +8 B (-0.2%, 3,956 B / 16,384 B, total: 24% used)

mirtoo

• kseg0_progmem: .data +4 B, .text +12 B (+0.0%, 67,952 B / 131,072 B, total: 52% used)
• kseg1_datamem: .data +4 B (+0.2%, 2,588 B / 32,256 B, total: 8% used)

qemu-armv8a

• Code: .rodata +16 B, .text.lib_fwrite_unlocked +64 B, .text.nsh_cmdparm.constprop.0 +16 B, .text.nsh_parse_command +12 B (+0.0%, 347,636 B)
• Data: .data.g_version +2 B (+0.0%, 77,476 B)

qemu-intel64

• Code: .text +108 B (+0.0%, 8,682,620 B)

rx65n-rsk2mb

• RAM: .data +4 B (+0.1%, 6,590 B / 262,144 B, total: 3% used)
• ROM: .rodata +16 B (+0.0%, 87,092 B / 2,097,152 B, total: 4% used)

s698pm-dkit

• Code: .text +144 B (+0.0%, 374,816 B)

stm32-nucleo-f103rb

• flash: .text +92 B (+0.3%, 34,752 B / 131,072 B, total: 27% used)