Fix false positive in isAbsPathOverlapping path boundary check

strings.HasPrefix without a trailing "/" causes false positives when
one path is a string prefix of another but not a path ancestor
(e.g. "/data/tsdb" vs "/data/tsdb-compactor/cache"). Append "/"
before comparing to enforce path segment boundaries.

Author: soya111

CHANGELOG.md

@@ -129,6 +129,7 @@
 * [ENHANCEMENT] Ingester: Add experimental file based Kafka consumer group offset tracking via flag `-ingest-storage.kafka.consumer-group-offset-commit-file-enforced`. #14110
 * [ENHANCEMENT] Store-gateway: Add "OOO" column to the tenant blocks page to indicate whether each block was created from out-of-order samples. #14283
 * [ENHANCEMENT] Ingester: Optimize ingestion from Kafka in clusters with mixed size tenants. #13924 #13961 #14302
+* [BUGFIX] Mimir: Fix false positive in filesystem path overlap detection when one path is a string prefix of another but not an ancestor directory (e.g. `/data/tsdb` vs `/data/tsdb-compactor/cache`). #14426
 * [BUGFIX] Mimir: Fix nil pointer dereference when `-target` is set to an empty string. #14381
 * [BUGFIX] API: Fixed web UI links not respecting `-server.path-prefix` configuration. #14090
 * [BUGFIX] Distributor: Fix issue where distributors didn't send custom values of native histograms. #13849

pkg/mimir/mimir.go

@@ -642,7 +642,9 @@ func isAbsPathOverlapping(firstAbsPath, secondAbsPath string) bool {
 	}
 
 	// The base directories are different, but they could still overlap if one is the child of the other one.
-	return strings.HasPrefix(firstAbsPath, secondAbsPath) || strings.HasPrefix(secondAbsPath, firstAbsPath)
+	// We append "/" to ensure we match on path segment boundaries and avoid false positives
+	// like "/data/tsdb" matching "/data/tsdb-compactor/cache".
+	return strings.HasPrefix(firstAbsPath, secondAbsPath+"/") || strings.HasPrefix(secondAbsPath, firstAbsPath+"/")
 }
 
 func (c *Config) registerServerFlagsWithChangedDefaultValues(fs *flag.FlagSet) {

pkg/mimir/mimir_test.go

@@ -807,6 +807,23 @@ func TestIsAbsPathOverlapping(t *testing.T) {
 			second:   "/path/to/more/data",
 			expected: false,
 		},
+		// Paths at different depths where one is a string prefix of the other
+		// but NOT a path ancestor (regression test for path boundary check).
+		{
+			first:    "/data/tsdb",
+			second:   "/data/tsdb-compactor/cache",
+			expected: false,
+		},
+		{
+			first:    "/data/tsdb-compactor/cache",
+			second:   "/data/tsdb",
+			expected: false,
+		},
+		{
+			first:    "/data/tsdb",
+			second:   "/data/tsdb/wal",
+			expected: true,
+		},
 	}
 
 	for _, testData := range tests {