fix: treat non-last segment corruption as hard error (C4)

Per design §3.2 line 704, tail corruption in a non-last WAL segment is
middle corruption, which must hard-fail recovery instead of being
silently truncated. The previous code in wal/recovery.go always returned
TailCorruptionError for CollectingFragments state or parse errors,
regardless of segment position. Recover then always truncated
segments[last], which could corrupt a valid last segment when the actual
corruption was in a middle segment.

Oracle bg_ef425776 flagged an additional failure mode: "wal/recover.go
总是对 segments[len(segments)-1] 调用截断，但 RecoverFromSegments 的
TailCorruptionError 可能来自非尾段".

Changes:
- wal/record_parser.go: add SegmentPath field to TailCorruptionError for
  diagnostics and defensive truncation target identification.
- wal/recovery.go:
  - ReplaySegmentFile now takes isLastSegment bool parameter.
  - When parse error or CollectingFragments occurs in non-last segment,
    return hard error. Uses %v (NOT %w) so IsTailCorruption returns false
    — otherwise errors.As would still find underlying TailCorruptionError
    through the %w chain and Recover would treat it as truncatable.
  - When in last segment, return TailCorruptionError with SegmentPath set.
  - RecoverFromSegments passes isLastSegment based on iteration index.
- wal/recover.go:
  - Use tce.SegmentPath as authoritative truncation target (defensive
    fallback to segments[last] if missing). After C4 fix, TailCorruptionError
    is only returned for last segment, so this is always segments[last]
    in practice.

Tests:
- wal/recovery_test.go: 4 unit tests for ReplaySegmentFile covering
  last/non-last × CollectingFragments/parse-error matrix. Existing direct
  ReplaySegmentFile calls updated to pass isLastSegment=true.
- wal/recover_test.go: 4 integration tests covering middle-segment
  CollectingFragments corruption (must hard-fail), middle-segment CRC
  corruption (must hard-fail), last-segment corruption in single-segment
  WAL (must truncate), last-segment corruption in multi-segment WAL
  (must truncate only last segment).

Verified: each new test fails on pre-fix code (non-last corruption
silently truncated valid last segment) and passes after the fix. Full
suite green including go test -race ./... .

Audit context: docs/audit-3.2.md C4 (Oracle-verified bg_ef425776).

wal/record_parser.go

@@ -10,12 +10,19 @@ import (
 // TailCorruptionError indicates that the WAL tail contains corrupt data
 // (bad CRC, unexpected non-zero padding bytes, etc.). Recovery may safely
 // truncate at the last valid record.
+//
+// SegmentPath is set by ReplaySegmentFile when it propagates the error,
+// so Recover can use it as the authoritative truncation target.
 type TailCorruptionError struct {
-	Offset int
-	Err    error
+	Offset      int
+	SegmentPath string
+	Err         error
 }
 
 func (e *TailCorruptionError) Error() string {
+	if e.SegmentPath != "" {
+		return fmt.Sprintf("wal: tail corruption in %s at offset %d: %v", e.SegmentPath, e.Offset, e.Err)
+	}
 	return fmt.Sprintf("wal: tail corruption at offset %d: %v", e.Offset, e.Err)
 }
 

wal/recover.go

@@ -1,6 +1,7 @@
 package wal
 
 import (
+	"errors"
 	"fmt"
 	"os"
 
@@ -59,19 +60,27 @@ func Recover(dir string, replayer BatchReplayer) (*RecoveryResult, error) {
 		}
 
 		if len(segments) > 0 {
-			lastSeg := segments[len(segments)-1]
-			lastCompleteBatchEnd, findErr := findLastCompleteBatchEnd(lastSeg.FilePath)
+			// After C4 fix, TailCorruptionError is only returned for the last
+			// segment. Use tce.SegmentPath as authoritative truncation target
+			// (defensive: fall back to segments[last] if missing).
+			corruptedPath := segments[len(segments)-1].FilePath
+			var tce *TailCorruptionError
+			if errors.As(err, &tce) && tce.SegmentPath != "" {
+				corruptedPath = tce.SegmentPath
+			}
+
+			lastCompleteBatchEnd, findErr := findLastCompleteBatchEnd(corruptedPath)
 			if findErr != nil {
 				return nil, fmt.Errorf("wal: recover: find truncation offset: %w", findErr)
 			}
 
 			// Phase 1: truncated segment is always segments[last], no
-			// trailing empty segments to clean up. C4 fix will need to
-			// identify trailing empties based on the actually-corrupted
-			// segment's index, which is not the same as segments[last].
+			// trailing empty segments to clean up. C4 fix means non-last
+			// segment corruption hard-fails above, so we never reach here
+			// with a non-last corrupted segment.
 			var emptyTrailing []string
 
-			if err := truncateAndPersist(lastSeg.FilePath, lastCompleteBatchEnd, dir, emptyTrailing); err != nil {
+			if err := truncateAndPersist(corruptedPath, lastCompleteBatchEnd, dir, emptyTrailing); err != nil {
 				// Per design §3.2 line 799: DB must NOT enter writable state.
 				return nil, fmt.Errorf("wal: recover: persist tail truncation: %w", err)
 			}

wal/recover_test.go

@@ -118,7 +118,7 @@ func TestRecoverWithTailCorruption(t *testing.T) {
 
 	// Verify the truncated file still parses cleanly.
 	replayer2 := &mockReplayer{}
-	_, parseErr := ReplaySegmentFile(filePath, 100, replayer2)
+	_, parseErr := ReplaySegmentFile(filePath, 100, true, replayer2)
 	if parseErr != nil {
 		t.Fatalf("replay after truncation: %v", parseErr)
 	}
@@ -451,3 +451,152 @@ func TestRecoverInvalidBatchNotTruncatable(t *testing.T) {
 			fi.Size(), int64(WalFileHeaderSize)+int64(len(rec)))
 	}
 }
+
+// -------- C4 integration regression guards --------
+
+// Regression guard for C4: middle segment CollectingFragments must hard-fail
+// Recover, NOT truncate the (valid) last segment. This is the key bug Oracle
+// flagged: "wal/recover.go 总是对 segments[len(segments)-1] 调用截断".
+func TestRecoverMiddleSegmentCorruptionHardFails(t *testing.T) {
+	dir := t.TempDir()
+	// segment-0: [Batch seq 0-1]   (2 entries → next=2)
+	// segment-1: [Batch seq 2-3] + [First][Middle no Last]  ← middle corruption
+	//            (2 complete entries → next=4 if recovery reached end)
+	// segment-2: [Batch seq 4-5]   (valid; never reached)
+	writeTestSegment(t, dir, 0, 0, [][]*WalEntry{
+		{makePutEntry("k0", "v0"), makePutEntry("k1", "v1")},
+	})
+
+	seg1Path := writeTestSegment(t, dir, 1, 2, [][]*WalEntry{
+		{makePutEntry("k2", "v2"), makePutEntry("k3", "v3")},
+	})
+	appendFileBytes(t, seg1Path, EncodePhysicalRecord(RecFirst, []byte("first-frag")))
+	appendFileBytes(t, seg1Path, EncodePhysicalRecord(RecMiddle, []byte("middle-frag")))
+
+	writeTestSegment(t, dir, 2, 4, [][]*WalEntry{
+		{makePutEntry("k4", "v4"), makePutEntry("k5", "v5")},
+	})
+
+	seg2Path := filepath.Join(dir, "segment-2.wal")
+	fiBefore, _ := os.Stat(seg2Path)
+
+	_, err := Recover(dir, &mockReplayer{})
+	if err == nil {
+		t.Fatal("expected Recover to fail on middle segment corruption")
+	}
+	if IsTailCorruption(err) {
+		t.Errorf("expected HARD error (not tail corruption) for middle segment; got %v", err)
+	}
+
+	// CRITICAL: segment-2 must NOT be truncated (it's completely valid).
+	fiAfter, _ := os.Stat(seg2Path)
+	if fiAfter.Size() != fiBefore.Size() {
+		t.Errorf("segment-2 was modified: before=%d after=%d (C4-2 regression)",
+			fiBefore.Size(), fiAfter.Size())
+	}
+}
+
+// Regression guard for C4 (parser corruption half): middle segment CRC
+// corruption must hard-fail Recover, NOT truncate the (valid) last segment.
+// Tests the path where ParseBlock returns TailCorruptionError and
+// ReplaySegmentFile converts it to hard error for non-last segment.
+func TestRecoverMiddleSegmentCRCCorruptionHardFails(t *testing.T) {
+	dir := t.TempDir()
+	writeTestSegment(t, dir, 0, 0, [][]*WalEntry{
+		{makePutEntry("k0", "v0"), makePutEntry("k1", "v1")},
+	})
+
+	seg1Path := writeTestSegment(t, dir, 1, 2, [][]*WalEntry{
+		{makePutEntry("k2", "v2"), makePutEntry("k3", "v3")},
+	})
+	// Append CRC-corrupted bytes (will fail DecodePhysicalRecord CRC check).
+	appendFileBytes(t, seg1Path, []byte{0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF})
+
+	writeTestSegment(t, dir, 2, 4, [][]*WalEntry{
+		{makePutEntry("k4", "v4"), makePutEntry("k5", "v5")},
+	})
+
+	seg2Path := filepath.Join(dir, "segment-2.wal")
+	fiBefore, _ := os.Stat(seg2Path)
+
+	_, err := Recover(dir, &mockReplayer{})
+	if err == nil {
+		t.Fatal("expected Recover to fail on middle segment CRC corruption")
+	}
+	if IsTailCorruption(err) {
+		t.Errorf("expected HARD error (not tail corruption); got %v", err)
+	}
+
+	fiAfter, _ := os.Stat(seg2Path)
+	if fiAfter.Size() != fiBefore.Size() {
+		t.Errorf("segment-2 modified: before=%d after=%d", fiBefore.Size(), fiAfter.Size())
+	}
+}
+
+// Regression guard for C4: single-segment tail corruption still truncates
+// correctly (existing behavior preserved).
+func TestRecoverLastSegmentCorruptionTruncatesCorrectly(t *testing.T) {
+	dir := t.TempDir()
+	batchA := []*WalEntry{makePutEntry("k0", "v0")}
+	batchB := []*WalEntry{makePutEntry("k1", "v1")}
+	filePath := writeTestSegment(t, dir, 0, 0, [][]*WalEntry{batchA, batchB})
+
+	encA, _ := EncodeWalBatch(0, batchA)
+	encB, _ := EncodeWalBatch(1, batchB)
+	endOfBatch1 := int64(WalFileHeaderSize) +
+		int64(PhysicalRecordHeaderSize+len(encA)) +
+		int64(PhysicalRecordHeaderSize+len(encB))
+
+	// Append partial fragments to last (only) segment.
+	appendFileBytes(t, filePath, EncodePhysicalRecord(RecFirst, []byte("first")))
+	appendFileBytes(t, filePath, EncodePhysicalRecord(RecMiddle, []byte("middle")))
+
+	result, err := Recover(dir, &mockReplayer{})
+	if err != nil {
+		t.Fatalf("Recover: %v", err)
+	}
+	if !result.Truncated {
+		t.Fatal("Truncated = false, want true for last-segment corruption")
+	}
+
+	fi, _ := os.Stat(filePath)
+	if fi.Size() != endOfBatch1 {
+		t.Errorf("file size = %d, want %d", fi.Size(), endOfBatch1)
+	}
+}
+
+// Regression guard for C4: multi-segment with last-segment corruption still
+// works correctly (the legitimate tail-truncation case).
+func TestRecoverMultiSegmentLastSegmentCorruptionTruncatesLast(t *testing.T) {
+	dir := t.TempDir()
+	writeTestSegment(t, dir, 0, 0, [][]*WalEntry{
+		{makePutEntry("k0", "v0")},
+	})
+	seg1Path := writeTestSegment(t, dir, 1, 1, [][]*WalEntry{
+		{makePutEntry("k1", "v1")},
+		{makePutEntry("k2", "v2")},
+	})
+
+	encA, _ := EncodeWalBatch(1, []*WalEntry{makePutEntry("k1", "v1")})
+	encB, _ := EncodeWalBatch(2, []*WalEntry{makePutEntry("k2", "v2")})
+	endOfBatch2 := int64(WalFileHeaderSize) +
+		int64(PhysicalRecordHeaderSize+len(encA)) +
+		int64(PhysicalRecordHeaderSize+len(encB))
+
+	// Append partial fragments to segment-1 (the LAST segment).
+	appendFileBytes(t, seg1Path, EncodePhysicalRecord(RecFirst, []byte("first")))
+	appendFileBytes(t, seg1Path, EncodePhysicalRecord(RecMiddle, []byte("middle")))
+
+	result, err := Recover(dir, &mockReplayer{})
+	if err != nil {
+		t.Fatalf("Recover: %v", err)
+	}
+	if !result.Truncated {
+		t.Fatal("Truncated = false, want true")
+	}
+
+	fi, _ := os.Stat(seg1Path)
+	if fi.Size() != endOfBatch2 {
+		t.Errorf("segment-1 size = %d, want %d (end of Batch 2)", fi.Size(), endOfBatch2)
+	}
+}

wal/recovery.go

@@ -100,13 +100,35 @@ func ReplayBatch(batch *WalBatch, expectedSequence uint64, replayer BatchReplaye
 }
 
 // ReplaySegmentFile replays all complete WAL batches from one segment file.
-func ReplaySegmentFile(filePath string, startSequence uint64, replayer BatchReplayer) (nextSequence uint64, err error) {
+//
+// isLastSegment controls how parse errors and CollectingFragments-at-end are
+// classified per design §3.2 line 704:
+//   - true: tail corruption (TailCorruptionError, truncatable by Recover)
+//   - false: hard corruption (plain error, Recover must hard-fail)
+func ReplaySegmentFile(filePath string, startSequence uint64, isLastSegment bool, replayer BatchReplayer) (nextSequence uint64, err error) {
 	nextSequence = startSequence
 	records, parseErr := ParseRecordsFromFile(filePath)
 	if parseErr != nil && !IsTailCorruption(parseErr) {
 		return nextSequence, fmt.Errorf("wal: parse segment records: %w", parseErr)
 	}
 
+	// Attach SegmentPath to parseErr for downstream diagnostics + truncation target.
+	if parseErr != nil {
+		var tce *TailCorruptionError
+		if errors.As(parseErr, &tce) {
+			tce.SegmentPath = filePath
+		}
+	}
+
+	// C4 fix: tail corruption in non-last segment is hard corruption per
+	// design §3.2 line 704. Use %v (NOT %w) so IsTailCorruption returns false
+	// — otherwise errors.As would still find the underlying *TailCorruptionError
+	// through the %w chain and Recover would treat it as truncatable.
+	if parseErr != nil && !isLastSegment {
+		return nextSequence, fmt.Errorf("wal: corruption in non-last segment %s (hard corruption): %v",
+			filePath, parseErr)
+	}
+
 	collector := NewFragmentCollector()
 	for _, record := range records {
 		if err := collector.Append(record.Type, record.Payload); err != nil {
@@ -131,10 +153,16 @@ func ReplaySegmentFile(filePath string, startSequence uint64, replayer BatchRepl
 		return nextSequence, parseErr
 	}
 	if collector.State() == FragmentCollecting {
-		return nextSequence, &TailCorruptionError{
-			Offset: 0,
-			Err:    errors.New("incomplete fragmented batch at segment tail"),
+		if isLastSegment {
+			return nextSequence, &TailCorruptionError{
+				Offset:      0,
+				SegmentPath: filePath,
+				Err:         errors.New("incomplete fragmented batch at segment tail"),
+			}
 		}
+		// Non-last segment with incomplete fragments = middle corruption.
+		// Plain error (no TailCorruptionError) so IsTailCorruption is false.
+		return nextSequence, fmt.Errorf("wal: incomplete fragmented batch in non-last segment %s (hard corruption)", filePath)
 	}
 
 	return nextSequence, nil
@@ -151,12 +179,13 @@ func RecoverFromSegments(dir string, recoverySegmentID uint64, replayer BatchRep
 	}
 
 	nextSequence = segments[0].StartSequence
-	for _, segment := range segments {
+	for i, segment := range segments {
 		if segment.StartSequence != nextSequence {
 			return nextSequence, fmt.Errorf("wal: segment start sequence %d does not match expected sequence %d", segment.StartSequence, nextSequence)
 		}
 
-		nextSequence, err = ReplaySegmentFile(segment.FilePath, nextSequence, replayer)
+		isLastSegment := i == len(segments)-1
+		nextSequence, err = ReplaySegmentFile(segment.FilePath, nextSequence, isLastSegment, replayer)
 		if err != nil {
 			if IsTailCorruption(err) {
 				return nextSequence, err

wal/recovery_test.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"math"
 	"os"
+	"path/filepath"
 	"reflect"
 	"strings"
 	"testing"
@@ -121,7 +122,7 @@ func TestReplaySegmentFileFull(t *testing.T) {
 	})
 
 	replayer := &mockReplayer{}
-	next, err := ReplaySegmentFile(filePath, 50, replayer)
+	next, err := ReplaySegmentFile(filePath, 50, true, replayer)
 	if err != nil {
 		t.Fatalf("ReplaySegmentFile: %v", err)
 	}
@@ -147,7 +148,7 @@ func TestReplaySegmentFileTailCorruption(t *testing.T) {
 	appendFileBytes(t, filePath, []byte{0x01, 0x02, 0x03})
 
 	replayer := &mockReplayer{}
-	next, err := ReplaySegmentFile(filePath, 70, replayer)
+	next, err := ReplaySegmentFile(filePath, 70, true, replayer)
 	if err == nil {
 		t.Fatal("ReplaySegmentFile succeeded, want tail corruption error")
 	}
@@ -250,3 +251,88 @@ func appendFileBytes(t *testing.T, filePath string, data []byte) {
 		t.Fatalf("Write corruption bytes: %v", err)
 	}
 }
+
+// -------- C4 regression guards: isLastSegment controls corruption classification --------
+
+// Regression guards for C4: isLastSegment controls whether CollectingFragments
+// at end is tail corruption (truncatable) or hard corruption (must hard-fail).
+
+func TestReplaySegmentFile_LastSegmentCollectingFragmentsIsTailCorruption(t *testing.T) {
+	dir := t.TempDir()
+	filePath := filepath.Join(dir, "segment-0.wal")
+	writeTestSegment(t, dir, 0, 0, [][]*WalEntry{
+		{makePutEntry("k", "v")},
+	})
+	// Append First + Middle (no Last) to leave collector in Collecting state.
+	appendFileBytes(t, filePath, EncodePhysicalRecord(RecFirst, []byte("first")))
+	appendFileBytes(t, filePath, EncodePhysicalRecord(RecMiddle, []byte("middle")))
+
+	_, err := ReplaySegmentFile(filePath, 0, true, &mockReplayer{})
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	if !IsTailCorruption(err) {
+		t.Errorf("expected TailCorruptionError for last segment, got: %v", err)
+	}
+	var tce *TailCorruptionError
+	if errors.As(err, &tce) {
+		if tce.SegmentPath != filePath {
+			t.Errorf("SegmentPath = %q, want %q", tce.SegmentPath, filePath)
+		}
+	}
+}
+
+func TestReplaySegmentFile_NonLastSegmentCollectingFragmentsIsHardError(t *testing.T) {
+	dir := t.TempDir()
+	filePath := filepath.Join(dir, "segment-0.wal")
+	writeTestSegment(t, dir, 0, 0, [][]*WalEntry{
+		{makePutEntry("k", "v")},
+	})
+	appendFileBytes(t, filePath, EncodePhysicalRecord(RecFirst, []byte("first")))
+	appendFileBytes(t, filePath, EncodePhysicalRecord(RecMiddle, []byte("middle")))
+
+	_, err := ReplaySegmentFile(filePath, 0, false, &mockReplayer{})
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	if IsTailCorruption(err) {
+		t.Errorf("expected HARD error for non-last segment, got TailCorruptionError: %v", err)
+	}
+}
+
+func TestReplaySegmentFile_LastSegmentParseErrorIsTailCorruption(t *testing.T) {
+	dir := t.TempDir()
+	filePath := filepath.Join(dir, "segment-0.wal")
+	writeTestSegment(t, dir, 0, 0, [][]*WalEntry{
+		{makePutEntry("k", "v")},
+	})
+	// Append corrupted bytes to trigger ParseBlock's CRC failure path.
+	appendFileBytes(t, filePath, []byte{0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF})
+
+	_, err := ReplaySegmentFile(filePath, 0, true, &mockReplayer{})
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	if !IsTailCorruption(err) {
+		t.Errorf("expected TailCorruptionError for last segment parse error, got: %v", err)
+	}
+}
+
+func TestReplaySegmentFile_NonLastSegmentParseErrorIsHardError(t *testing.T) {
+	dir := t.TempDir()
+	filePath := filepath.Join(dir, "segment-0.wal")
+	writeTestSegment(t, dir, 0, 0, [][]*WalEntry{
+		{makePutEntry("k", "v")},
+	})
+	appendFileBytes(t, filePath, []byte{0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF})
+
+	_, err := ReplaySegmentFile(filePath, 0, false, &mockReplayer{})
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	// Per C4 fix: non-last segment parser corruption is hard error (NOT TailCorruption).
+	// Implemented via %v (not %w) so errors.As cannot find underlying TailCorruptionError.
+	if IsTailCorruption(err) {
+		t.Errorf("expected HARD error for non-last segment parse error, got TailCorruptionError: %v", err)
+	}
+}