Use sys.stderr.write() to fix stderr interleaving under make -jN

[aditya-qti]

Example interleaved stderr:

/home/patchwise/build/arch/arm64/boot/dts/microchip/sparx5_pcb134_emmc.dtb: syscon@600000000 (microchip,sparx5-cpu-syscon): mux-controller:#mux-control-cells: 1 was expected
	from schema $id: http://devicetree.org/schemas/soc/microchip/microchip,sparx5-cpu-syscon.yaml/home/patchwise/build/arch/arm64/boot/dts/microchip/sparx5_pcb134.dtb: syscon@600000000 (microchip,sparx5-cpu-syscon): mux-controller:#mux-control-cells: 1 was expected
	from schema $id: http://devicetree.org/schemas/soc/microchip/microchip,sparx5-cpu-syscon.yaml

This issue was found when running make -j$(nproc) dtbs_check in PatchWise (automates patch review and static analysis):

https://github.com/qualcomm/PatchWise/blob/main/patchwise/patch_review/static_analysis/dtbs_check.py#L36-L51

This makes it difficult for automations to find the diff between before and after errors of the current commit.

The root cause is that on Python's default line-buffered sys.stderr, print() makes 2 write() syscalls when
the input is a multi-line string, and a sibling process' write() can execute between those two syscalls.

single-line print() -> 1 write() syscall.
ex: print("msg\n") -> 1 write()

multi-line print() -> 2 write() syscalls.
ex: print(" line1 \n line2 \n line3 ") -> 2 write() syscalls

• one for "line1 \n line2 \n line3"
• one for "\n" that print() adds at the end

Python's sys.stderr is line-buffered by default, so the pending buffer gets flushed when "\n" is seen.

p1                                             p2
write(" line1 \n line2 \n line3 ")
                                            write("msg\n")
write("\n")

Output:

 line1
 line2
 line3 msg

This can be proven by running strace on something like this:

import sys


def banner(label):
    sys.stderr.write(f"=== {label} ===\n")
    sys.stderr.flush()


banner("print(single-line)")
print("ABCDEFGH" * 4, file=sys.stderr)
sys.stderr.flush()

banner("print(multi-line)")
print("LINE1-XX\nLINE2-XX\nLINE3-XX", file=sys.stderr)
sys.stderr.flush()

banner("write(multi-line + '\\n')")
sys.stderr.write("LINE1-XX\nLINE2-XX\nLINE3-XX" + "\n")
sys.stderr.flush()

$ strace -f -e trace=write -e signal=none -o trace_print.strace python3 trace_print.py 2>trace_print.out
$ grep 'write(2,' trace_print.strace
2804383 write(2, "=== print(single-line) ===\n", 27) = 27
2804383 write(2, "ABCDEFGHABCDEFGHABCDEFGHABCDEFGH"..., 33) = 33
2804383 write(2, "=== print(multi-line) ===\n", 26) = 26
2804383 write(2, "LINE1-XX\nLINE2-XX\nLINE3-XX", 26) = 26
2804383 write(2, "\n", 1)               = 1
2804383 write(2, "=== write(multi-line + '\\n') ==="..., 33) = 33
2804383 write(2, "LINE1-XX\nLINE2-XX\nLINE3-XX\n", 27) = 27

$ cat trace_print.out
=== print(single-line) ===
ABCDEFGHABCDEFGHABCDEFGHABCDEFGH
=== print(multi-line) ===
LINE1-XX
LINE2-XX
LINE3-XX
=== write(multi-line + '\n') ===
LINE1-XX
LINE2-XX
LINE3-XX

Fix the issue by using sys.stderr.write(text + "\n") instead of print(text, file=sys.stderr),
since sys.stderr.write(multi_line_text) makes a single write() call.

[robherring]

Can we just fix the multi-line cases and leave the other print()'s alone.

[aditya-qti]

single-line print() -> 1 write() syscall.
ex: print("msg\n") -> 1 write()

I meant to write print("msg") here :(

[robherring]

Please squash the reverted parts so I can review it.