Detect archive type by file signature when extension is wrong or missing

.golangci.yml

@@ -24,6 +24,9 @@ linters:
         path: chardet\.go
 
   settings:
+    gomoddirectives:
+      replace-allow-list:
+        - github.com/kdomanski/iso9660
     gosec:
       excludes:
         - G304

files.go

@@ -352,11 +352,32 @@ func ExtractFile(xFile *XFile) (size uint64, filesList, archiveList []string, er
 
 	for _, ext := range extension2function {
 		if strings.HasSuffix(sName, ext.Extension) {
-			return ext.Extract(xFile)
+			size, filesList, archiveList, err = ext.Extract(xFile)
+			if err == nil {
+				return size, filesList, archiveList, nil
+			}
+
+			// Extension matched but extraction failed; try signature detection as fallback.
+			break
 		}
 	}
 
-	return 0, nil, nil, fmt.Errorf("%w: %s", ErrUnknownArchiveType, xFile.FilePath)
+	// Fall back to file signature (magic number) detection.
+	xFile.Debugf("falling back to signature detection for %s (extension error: %v)", xFile.FilePath, err)
+
+	extractFn, sigErr := detectBySignature(xFile.FilePath)
+	if sigErr != nil {
+		extErr := &ExtractError{}
+		if err != nil {
+			extErr.Errs = append(extErr.Errs, err)
+		}
+
+		extErr.Errs = append(extErr.Errs, sigErr)
+
+		return 0, nil, nil, extErr
+	}
+
+	return extractFn(xFile)
 }
 
 // MoveFiles relocates files then removes the folder they were in.

magic.go

@@ -0,0 +1,108 @@
+package xtractr
+
+/* Code to detect archive types by file signatures (magic numbers). */
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+)
+
+// signature maps a byte pattern at a specific offset to an extract function.
+type signature struct {
+	// Offset is the byte offset where the magic bytes are expected.
+	Offset int
+	// Magic is the byte sequence to match at Offset.
+	Magic []byte
+	// Extract function for this signature.
+	Extract Interface
+}
+
+// maxSignatureRead is the maximum number of bytes to read for signature detection.
+// This is enough for ISO9660 detection at offset 0x9001 + 5 bytes for "CD001".
+const maxSignatureRead = 0x9006
+
+// signatureTable maps file signatures (magic numbers) to their corresponding extract functions.
+//
+//nolint:gochecknoglobals
+var signatureTable = []signature{
+	// RAR v5 (longer match first).
+	{Offset: 0, Magic: []byte{0x52, 0x61, 0x72, 0x21, 0x1A, 0x07, 0x01, 0x00}, Extract: ExtractRAR},
+	// RAR v4.
+	{Offset: 0, Magic: []byte{0x52, 0x61, 0x72, 0x21, 0x1A, 0x07, 0x00}, Extract: ExtractRAR},
+	// 7-Zip.
+	{Offset: 0, Magic: []byte{0x37, 0x7A, 0xBC, 0xAF, 0x27, 0x1C}, Extract: Extract7z},
+	// ZIP (PK\x03\x04).
+	{Offset: 0, Magic: []byte{0x50, 0x4B, 0x03, 0x04}, Extract: ChngInt(ExtractZIP)},
+	// Gzip.
+	{Offset: 0, Magic: []byte{0x1F, 0x8B}, Extract: ChngInt(ExtractGzip)},
+	// Bzip2 (BZh).
+	{Offset: 0, Magic: []byte{0x42, 0x5A, 0x68}, Extract: ChngInt(ExtractBzip)},
+	// XZ.
+	{Offset: 0, Magic: []byte{0xFD, 0x37, 0x7A, 0x58, 0x5A, 0x00}, Extract: ChngInt(ExtractXZ)},
+	// Zstandard.
+	{Offset: 0, Magic: []byte{0x28, 0xB5, 0x2F, 0xFD}, Extract: ChngInt(ExtractZstandard)},
+	// LZ4.
+	{Offset: 0, Magic: []byte{0x04, 0x22, 0x4D, 0x18}, Extract: ChngInt(ExtractLZ4)},
+	// LZMA.
+	{Offset: 0, Magic: []byte{0x5D, 0x00, 0x00}, Extract: ChngInt(ExtractLZMA)},
+	// Brotli.
+	{Offset: 0, Magic: []byte{0xCE, 0xB2, 0xCF, 0x81}, Extract: ChngInt(ExtractBrotli)},
+	// AR / DEB ("!<arch>\n").
+	{Offset: 0, Magic: []byte{0x21, 0x3C, 0x61, 0x72, 0x63, 0x68, 0x3E, 0x0A}, Extract: ChngInt(ExtractAr)},
+	// RPM.
+	{Offset: 0, Magic: []byte{0xED, 0xAB, 0xEE, 0xDB}, Extract: ChngInt(ExtractRPM)},
+	// ISO9660 at offset 0x8001.
+	{Offset: 0x8001, Magic: []byte{0x43, 0x44, 0x30, 0x30, 0x31}, Extract: ChngInt(ExtractISO)}, //nolint:mnd
+	// ISO9660 at offset 0x8801.
+	{Offset: 0x8801, Magic: []byte{0x43, 0x44, 0x30, 0x30, 0x31}, Extract: ChngInt(ExtractISO)}, //nolint:mnd
+	// ISO9660 at offset 0x9001.
+	{Offset: 0x9001, Magic: []byte{0x43, 0x44, 0x30, 0x30, 0x31}, Extract: ChngInt(ExtractISO)}, //nolint:mnd
+}
+
+// detectBySignature reads the first bytes of a file and attempts to match
+// a known file signature (magic number) to determine the archive type.
+func detectBySignature(filePath string) (Interface, error) {
+	file, err := os.Open(filePath)
+	if err != nil {
+		return nil, fmt.Errorf("opening file for signature detection: %w", err)
+	}
+	defer file.Close()
+
+	stat, err := file.Stat()
+	if err != nil {
+		return nil, fmt.Errorf("stat file for signature detection: %w", err)
+	}
+
+	readSize := min(stat.Size(), int64(maxSignatureRead))
+
+	buf := make([]byte, readSize)
+
+	n, err := file.Read(buf)
+	if err != nil {
+		return nil, fmt.Errorf("reading file for signature detection: %w", err)
+	}
+
+	buf = buf[:n]
+
+	for _, sig := range signatureTable {
+		end := sig.Offset + len(sig.Magic)
+		if end > len(buf) {
+			continue
+		}
+
+		if bytes.Equal(buf[sig.Offset:end], sig.Magic) {
+			return sig.Extract, nil
+		}
+	}
+
+	return nil, fmt.Errorf("%w: %s", ErrUnknownArchiveType, filePath)
+}
+
+// IsArchiveFileByContent returns true if the provided file path contains
+// a recognized archive file signature. Unlike IsArchiveFile, this reads
+// the actual file content rather than relying on the file extension.
+func IsArchiveFileByContent(path string) bool {
+	extractFn, err := detectBySignature(path)
+	return err == nil && extractFn != nil
+}