Compress records before checking segment size (#8501)

Right now a new segment might be created unnecessarily if the
uncompressed record would not fit, but after compression (typically
reducing record size in half) it would.

Signed-off-by: Chris Marchbanks <csmarchbanks@gmail.com>

tsdb/wal/wal.go

@@ -613,18 +613,8 @@ func (w *WAL) log(rec []byte, final bool) error {
 			return err
 		}
 	}
-	// If the record is too big to fit within the active page in the current
-	// segment, terminate the active segment and advance to the next one.
-	// This ensures that records do not cross segment boundaries.
-	left := w.page.remaining() - recordHeaderSize                                   // Free space in the active page.
-	left += (pageSize - recordHeaderSize) * (w.pagesPerSegment() - w.donePages - 1) // Free pages in the active segment.
-
-	if len(rec) > left {
-		if err := w.nextSegment(); err != nil {
-			return err
-		}
-	}
 
+	// Compress the record before calculating if a new segment is needed.
 	compressed := false
 	if w.compress && len(rec) > 0 {
 		// The snappy library uses `len` to calculate if we need a new buffer.
@@ -638,6 +628,18 @@ func (w *WAL) log(rec []byte, final bool) error {
 		}
 	}
 
+	// If the record is too big to fit within the active page in the current
+	// segment, terminate the active segment and advance to the next one.
+	// This ensures that records do not cross segment boundaries.
+	left := w.page.remaining() - recordHeaderSize                                   // Free space in the active page.
+	left += (pageSize - recordHeaderSize) * (w.pagesPerSegment() - w.donePages - 1) // Free pages in the active segment.
+
+	if len(rec) > left {
+		if err := w.nextSegment(); err != nil {
+			return err
+		}
+	}
+
 	// Populate as many pages as necessary to fit the record.
 	// Be careful to always do one pass to ensure we write zero-length records.
 	for i := 0; i == 0 || len(rec) > 0; i++ {