querylog: imp docs

internal/querylog/qlogfile.go

@@ -175,31 +175,23 @@ type tsSearchState struct {
 	depth int
 }
 
-// maxSeekDepth is the maximum allowed search iterations.
-const maxSeekDepth = 100
+// maxSearchDepth is the maximum number of search iterations.
+const maxSearchDepth = 100
 
-// seekTSStep performs one iteration of the binary search over the qlog file.
+// seekTSStep performs one iteration of the binary search over the qlog file.  l
+// and st must not be nil.
 func (q *qLogFile) seekTSStep(
 	ctx context.Context,
 	l *slog.Logger,
 	timestamp int64,
 	st *tsSearchState,
 ) (found bool, err error) {
-	// Get the line at the specified position.
-	line, lineIdx, lineEndIdx, err := q.readProbeLine(st.probe)
+	line, lineIdx, lineEndIdx, err := q.readAndValidateProbe(st, timestamp)
 	if err != nil {
+		// Don't wrap the error, because it's informative enough as is.
 		return false, err
 	}
 
-	// Check if the line index is invalid.
-	err = q.validateQLogLineIdx(lineIdx, st.lastProbeLineIdx, timestamp, st.fileSize)
-	if err != nil {
-		return false, err
-	}
-
-	// Save the last probed index.
-	st.lastProbeLineIdx = lineIdx
-
 	// Get the timestamp from the query log record.
 	ts := readQLogTimestamp(ctx, l, line)
 	if ts == 0 {
@@ -234,7 +226,7 @@ func (q *qLogFile) seekTSStep(
 	st.probe = st.start + (st.end-st.start)/2
 
 	st.depth++
-	if st.depth >= maxSeekDepth {
+	if st.depth >= maxSearchDepth {
 		return false, fmt.Errorf(
 			"looking up timestamp %d in %q: depth %d too high: %w",
 			timestamp,
@@ -247,6 +239,29 @@ func (q *qLogFile) seekTSStep(
 	return false, nil
 }
 
+// readAndValidateProbe reads the probe line and validates the line index.
+func (q *qLogFile) readAndValidateProbe(
+	st *tsSearchState,
+	timestamp int64,
+) (line string, lineIdx, lineEndIdx int64, err error) {
+	// Get the line at the specified position.
+	line, lineIdx, lineEndIdx, err = q.readProbeLine(st.probe)
+	if err != nil {
+		return "", 0, 0, fmt.Errorf("reading probe line: %w", err)
+	}
+
+	// Check if the line index is invalid.
+	err = q.validateQLogLineIdx(lineIdx, st.lastProbeLineIdx, timestamp, st.fileSize)
+	if err != nil {
+		return "", 0, 0, fmt.Errorf("validating line index: %w", err)
+	}
+
+	// Save the last probed index.
+	st.lastProbeLineIdx = lineIdx
+
+	return line, lineIdx, lineEndIdx, nil
+}
+
 // SeekStart changes the current position to the end of the file.  Please note,
 // that we're reading query log in the reverse order and that's why log start
 // is actually the end of file.