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encode: prevent appending .0 to scientific notation to le bucket labels #261

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encode: prevent appending .0 to scientific notation to le bucket labels #261

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6 changes: 5 additions & 1 deletion src/cmt_encode_prometheus.c
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Original file line number Diff line number Diff line change
Expand Up @@ -373,7 +373,11 @@ static cfl_sds_t bucket_value_to_string(double val)
len = snprintf(str, 64, "%g", val);
cfl_sds_len_set(str, len);

if (!strchr(str, '.')) {
/*
* Append .0 only when there is no decimal point and the number
* is not in scientific notation.
*/
if (!strchr(str, '.') && !strchr(str, 'e')) {
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Copilot AI Apr 2, 2026

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The check for scientific notation only looks for lowercase 'e'. While %g typically uses 'e', this becomes fragile if the formatting ever changes (e.g., %G) or if a platform emits an uppercase exponent. Consider using a character-class check (e.g., treating either 'e' or 'E' as scientific notation) so .0 is never appended to exponent-form values.

Suggested change
if (!strchr(str, '.') && !strchr(str, 'e')) {
if (!strchr(str, '.') && !strchr(str, 'e') && !strchr(str, 'E')) {

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cfl_sds_cat_safe(&str, ".0", 2);
Comment on lines +376 to 381
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Copilot AI Apr 2, 2026

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This change fixes a Prometheus parsing failure for large bucket boundaries, but there doesn’t appear to be a regression test covering scientific-notation le (e.g., a histogram bucket at >= 1e6 that should encode as 1e+06 and remain parseable). Adding a test that encodes and then decodes Prometheus text (or validates the encoded string) would prevent this from reoccurring.

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}
Comment on lines 373 to 382
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Copilot AI Apr 2, 2026

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bucket_value_to_string() will still append .0 to non-finite printf outputs like inf/nan (which contain neither . nor e), producing values like inf.0/nan.0 that are not parseable as floats in Prometheus label values. Since histogram buckets can be user-provided without an isfinite validation, consider explicitly handling non-finite val (e.g., reject, or map infinities to +Inf/-Inf, and avoid appending .0 for nan/inf).

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40 changes: 40 additions & 0 deletions tests/histogram.c
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Original file line number Diff line number Diff line change
Expand Up @@ -109,6 +109,45 @@ static void prometheus_encode_test(struct cmt *cmt)
cmt_encode_prometheus_destroy(buf);
}

void test_histogram_scientific_notation_bucket_label()
{
uint64_t ts;
cfl_sds_t buf;
struct cmt *cmt;
struct cmt_histogram *h;
struct cmt_histogram_buckets *buckets;

cmt_initialize();

ts = 0;
cmt = cmt_create();
TEST_CHECK(cmt != NULL);

buckets = cmt_histogram_buckets_create(1, 100000000.0);
TEST_CHECK(buckets != NULL);

h = cmt_histogram_create(cmt,
"cm", "encoding", "scientific_bucket",
"Histogram scientific bucket label",
buckets,
0, NULL);
TEST_CHECK(h != NULL);

cmt_histogram_observe(h, ts, 42.0, 0, NULL);

buf = cmt_encode_prometheus_create(cmt, CMT_TRUE);
TEST_CHECK(buf != NULL);
if (buf != NULL) {
TEST_CHECK(strstr(buf,
"cm_encoding_scientific_bucket_bucket{le=\"1e+08\"} 1 0") != NULL);
TEST_CHECK(strstr(buf,
"cm_encoding_scientific_bucket_bucket{le=\"1e+08.0\"}") == NULL);
cmt_encode_prometheus_destroy(buf);
}

cmt_destroy(cmt);
}


void test_histogram()
{
Expand Down Expand Up @@ -206,6 +245,7 @@ void test_set_defaults()
}

TEST_LIST = {
{"scientific_notation_bucket_label", test_histogram_scientific_notation_bucket_label},
{"histogram" , test_histogram},
{"set_defaults", test_set_defaults},
{ 0 }
Expand Down
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