strconv: string interpolation with '${f:54.40f}' leads to a MemorySanitizer: use-of-uninitialized-value error

[spytheman]

V doctor:

V full version: V 0.4.9 734bb00.3ed799e
OS: linux, Ubuntu 20.04.6 LTS
Processor: 4 cpus, 64bit, little endian, Intel(R) Core(TM) i3-3225 CPU @ 3.30GHz

getwd: /home/delian/code/v
vexe: /home/delian/code/v/v
vexe mtime: 2025-01-03 23:59:54

vroot: OK, value: /home/delian/code/v
VMODULES: OK, value: /home/delian/.vmodules
VTMP: OK, value: /tmp/v_1000

Git version: git version 2.47.1
Git vroot status: weekly.2024.53-19-g3ed799ef
.git/config present: true

CC version: cc (Ubuntu 10.5.0-1ubuntu1~20.04) 10.5.0
emcc version: N/A
thirdparty/tcc status: thirdparty-linux-amd64 0134e9b9

What did you do?
./v -g -o vdbg cmd/v && ./vdbg -keepc -cg -keepc -gc none -cc clang-18 -cflags -fsanitize=memory x_test.v && x_test

fn diff(actual f64, expected f64) string {
    C.printf(c' printf   actual: %054.54g\n', actual)
    C.printf(c' printf expected: %054.54g\n', expected)
    return '\n  actual:${actual:54.40f}\nexpected:${expected:54.40f}\n    diff:${actual - expected:54.40f}'
}

fn test_diff() {
    actual__ := -0.68849533748148505907238359213806688785552978515625
    expected := -0.68849533748148505907238359213806688785552978515625
    s := diff(actual__, expected)
    dump(s)
    assert true
}

What did you see?

==753252==WARNING: MemorySanitizer: use-of-uninitialized-value
/usr/bin/llvm-symbolizer-18: /usr/local/lib/libcurl.so.4: no version information available (required by /usr/bin/llvm-symbolizer-18)
    #0 0x555555645c90 in strconv__f64_to_str_lnd1 /tmp/v_1000/x_test.tmp.c:3920:9
    #1 0x555555646c3b in strconv__format_fl /tmp/v_1000/x_test.tmp.c:3949:15
    #2 0x555555693f55 in StrIntpData_process_str_intp_data /tmp/v_1000/x_test.tmp.c:7743:19
    #3 0x55555564c901 in str_intp /tmp/v_1000/x_test.tmp.c:7994:4
    #4 0x5555556ab1e9 in main__diff /tmp/v_1000/x_test.tmp.c:9053:9
    #5 0x5555556ab370 in main__test_diff /tmp/v_1000/x_test.tmp.c:9058:13
    #6 0x5555556ac837 in main /tmp/v_1000/x_test.tmp.c:9139:3
    #7 0x7ffff7be1082 in __libc_start_main /build/glibc-LcI20x/glibc-2.31/csu/../csu/libc-start.c:308:16
    #8 0x55555558b39d in _start (/home/delian/code/v/x_test+0x3739d) (BuildId: 939d919dc452a11c3d737f0f911fa027413209ff)

SUMMARY: MemorySanitizer: use-of-uninitialized-value /tmp/v_1000/x_test.tmp.c:3920:9 in strconv__f64_to_str_lnd1
Exiting

What did you expect to see?

A test program that runs without errors.

With -g, the stack trace becomes:

/usr/bin/llvm-symbolizer-18: /usr/local/lib/libcurl.so.4: no version information available (required by /usr/bin/llvm-symbolizer-18)
    #0 0x555555645c90 in strconv__f64_to_str_lnd1 /home/delian/code/v/vlib/strconv/format_mem.c.v:287:9
    #1 0x555555646c3b in strconv__format_fl /home/delian/code/v/vlib/strconv/format_mem.c.v:318:15
    #2 0x555555693f55 in StrIntpData_process_str_intp_data /home/delian/code/v/vlib/builtin/string_interpolation.v:444:19
    #3 0x55555564c901 in str_intp /home/delian/code/v/vlib/builtin/string_interpolation.v:706:4
    #4 0x5555556ab1e9 in main__diff /home/delian/code/v/x_test.v:4:9
    #5 0x5555556ab370 in main__test_diff /home/delian/code/v/x_test.v:10:13
    #6 0x5555556ac837 in main /tmp/v_1000/x_test.tmp.c:17219:3
    #7 0x7ffff7be1082 in __libc_start_main /build/glibc-LcI20x/glibc-2.31/csu/../csu/libc-start.c:308:16
    #8 0x55555558b39d in _start (/home/delian/code/v/x_test+0x3739d) (BuildId: b871f77ea6767383b0c54f7963b09b8cc8cd51bc)

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_{Huly®: V_0.6-21790}

[spytheman]

Note changing line 212 of vlib/strconv/format_mem.c.v like this:

-               mut res := []u8{len: exp + 40, init: 0}
+               mut res := []u8{len: exp + 50, init: 0}

makes the test program pass.