Direct usage of flb_sds_alloc as size argument for flb_sds_snprintf

[braydonk]

Bug Report

Describe the bug
There are numerous spots throughout the codebase where flb_sds_alloc is used as the size argument for flb_sds_snprintf. flb_sds_alloc gets the alloc field of struct flb_sds, which does not include the null terminator, however vsnprintf expects a size argument that does include the null terminator. This leads to a potential bug where the result of flb_sds_snprintf could include one less character than expected.

To Reproduce
In a random test file that included flb_sds.h, I wrote the following test:

static void test_snprintf()
{
    flb_sds_t s = flb_sds_create_size(4);
    flb_sds_snprintf(&s, flb_sds_alloc(s)+1, "%s", "test");
    printf("\n%s\n", s);
    TEST_ASSERT(strcmp(s, "test") == 0);
}

The result was this:

$ /usr/local/google/home/braydonk/Git/fluent-bit/build/bin/flb-it-csv snprintf
Test snprintf...                                
test
[ OK ]
SUCCESS: All unit tests have passed.

When I change it to not adjusting the size (i.e. instead of flb_sds_alloc(s)+1 it's just flb_sds_alloc(s)):

braydonk@bk:~/Git/fluent-bit/build/bin$ /usr/local/google/home/braydonk/Git/fluent-bit/build/bin/flb-it-csv snprintf
Test snprintf...                                
tes
[ FAILED ]
  csv.c:169: Check strcmp(s, "test") == 0... failed
FAILED: 1 of 1 unit tests has failed.

Additional context
First discovered by @jefferbrecht here: #9779 (comment)

Searching in this repo for flb_sds_snprintf usage has some examples right on the first page, but here is a quick list of a few spots doing it as an example:

fluent-bit/plugins/out_es/es.c

Line 366 in 68f1887

index_len = flb_sds_snprintf(&j_index,

fluent-bit/plugins/out_azure_logs_ingestion/azure_logs_ingestion_conf.c

Line 102 in 68f1887

flb_sds_snprintf(&ctx->auth_url, flb_sds_alloc(ctx->auth_url),

fluent-bit/plugins/out_azure_kusto/azure_kusto.c

Line 152 in 68f1887

flb_sds_snprintf(&body, flb_sds_alloc(body),

This is unlikely to be an issue of safety/exposure to a vulnerability thanks to snprintf safely discarding any characters past <size argument> - 1.