archive/tar: isolate regular and sparse file handling as methods

Factor out the regular file handling logic into handleRegularFile
from nextHeader. We will need to reuse this logic when fixing #15573
in a future CL.

Factor out the sparse file handling logic into handleSparseFile.
Currently this logic is split between nextHeader (for GNU sparse
files) and Next (for PAX sparse files). Instead, we move this
related code into a single method.

There is no overall logic change. Thus, no unit tests.

Updates #15573 #15564

Change-Id: I3b8270d8b4e080e77d6c0df6a123d677c82cc466
Reviewed-on: https://go-review.googlesource.com/27454Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/archive/tar/reader.go

@@ -110,6 +110,7 @@ func (tr *Reader) Next() (*Header, error) {
 	}
 
 	var hdr *Header
+	var rawHdr *block
 	var extHdrs map[string]string
 
 	// Externally, Next iterates through the tar archive as if it is a series of
@@ -124,7 +125,12 @@ loop:
 			return nil, tr.err
 		}
 
-		hdr = tr.readHeader()
+		hdr, rawHdr = tr.readHeader()
+		if tr.err != nil {
+			return nil, tr.err
+		}
+
+		tr.err = tr.handleRegularFile(hdr)
 		if tr.err != nil {
 			return nil, tr.err
 		}
@@ -161,21 +167,20 @@ loop:
 			}
 			continue loop // This is a meta header affecting the next header
 		default:
+			// The old GNU sparse format is handled here since it is technically
+			// just a regular file with additional attributes.
+
+			// TODO(dsnet): We should handle errors reported by mergePAX.
 			mergePAX(hdr, extHdrs)
 
-			// Check for a PAX format sparse file
-			sp, err := tr.checkForGNUSparsePAXHeaders(hdr, extHdrs)
-			if err != nil {
-				tr.err = err
-				return nil, err
-			}
-			if sp != nil {
-				// Current file is a PAX format GNU sparse file.
-				// Set the current file reader to a sparse file reader.
-				tr.curr, tr.err = newSparseFileReader(tr.curr, sp, hdr.Size)
-				if tr.err != nil {
-					return nil, tr.err
-				}
+			// TODO(dsnet): The extended headers may have updated the size.
+			// Thus, we must setup the regFileReader again here.
+			//
+			// See golang.org/issue/15573
+
+			tr.err = tr.handleSparseFile(hdr, rawHdr, extHdrs)
+			if tr.err != nil {
+				return nil, tr.err
 			}
 			break loop // This is a file, so stop
 		}
@@ -183,6 +188,54 @@ loop:
 	return hdr, nil
 }
 
+// handleRegularFile sets up the current file reader and padding such that it
+// can only read the following logical data section. It will properly handle
+// special headers that contain no data section.
+func (tr *Reader) handleRegularFile(hdr *Header) error {
+	nb := hdr.Size
+	if isHeaderOnlyType(hdr.Typeflag) {
+		nb = 0
+	}
+	if nb < 0 {
+		return ErrHeader
+	}
+
+	tr.pad = -nb & (blockSize - 1) // blockSize is a power of two
+	tr.curr = &regFileReader{r: tr.r, nb: nb}
+	return nil
+}
+
+// handleSparseFile checks if the current file is a sparse format of any type
+// and sets the curr reader appropriately.
+func (tr *Reader) handleSparseFile(hdr *Header, rawHdr *block, extHdrs map[string]string) error {
+	var sp []sparseEntry
+	var err error
+	if hdr.Typeflag == TypeGNUSparse {
+		var p parser
+		hdr.Size = p.parseNumeric(rawHdr.GNU().RealSize())
+		if p.err != nil {
+			return p.err
+		}
+
+		sp = tr.readOldGNUSparseMap(rawHdr)
+		if tr.err != nil {
+			return tr.err
+		}
+	} else {
+		sp, err = tr.checkForGNUSparsePAXHeaders(hdr, extHdrs)
+		if err != nil {
+			return err
+		}
+	}
+
+	// If sp is non-nil, then this is a sparse file.
+	// Note that it is possible for len(sp) to be zero.
+	if sp != nil {
+		tr.curr, err = newSparseFileReader(tr.curr, sp, hdr.Size)
+	}
+	return err
+}
+
 // checkForGNUSparsePAXHeaders checks the PAX headers for GNU sparse headers. If they are found, then
 // this function reads the sparse map and returns it. Unknown sparse formats are ignored, causing the file to
 // be treated as a regular file.
@@ -532,35 +585,36 @@ func (tr *Reader) skipUnread() error {
 }
 
 // readHeader reads the next block header and assumes that the underlying reader
-// is already aligned to a block boundary.
+// is already aligned to a block boundary. It returns the raw block of the
+// header in case further processing is required.
 //
 // The err will be set to io.EOF only when one of the following occurs:
 //	* Exactly 0 bytes are read and EOF is hit.
 //	* Exactly 1 block of zeros is read and EOF is hit.
 //	* At least 2 blocks of zeros are read.
-func (tr *Reader) readHeader() *Header {
+func (tr *Reader) readHeader() (*Header, *block) {
 	if _, tr.err = io.ReadFull(tr.r, tr.blk[:]); tr.err != nil {
-		return nil // io.EOF is okay here
+		return nil, nil // io.EOF is okay here
 	}
 
 	// Two blocks of zero bytes marks the end of the archive.
 	if bytes.Equal(tr.blk[:], zeroBlock[:]) {
 		if _, tr.err = io.ReadFull(tr.r, tr.blk[:]); tr.err != nil {
-			return nil // io.EOF is okay here
+			return nil, nil // io.EOF is okay here
 		}
 		if bytes.Equal(tr.blk[:], zeroBlock[:]) {
 			tr.err = io.EOF
 		} else {
 			tr.err = ErrHeader // zero block and then non-zero block
 		}
-		return nil
+		return nil, nil
 	}
 
 	// Verify the header matches a known format.
 	format := tr.blk.GetFormat()
 	if format == formatUnknown {
 		tr.err = ErrHeader
-		return nil
+		return nil, nil
 	}
 
 	var p parser
@@ -605,47 +659,12 @@ func (tr *Reader) readHeader() *Header {
 		}
 	}
 
-	nb := hdr.Size
-	if isHeaderOnlyType(hdr.Typeflag) {
-		nb = 0
-	}
-	if nb < 0 {
-		tr.err = ErrHeader
-		return nil
-	}
-
-	// Set the current file reader.
-	tr.pad = -nb & (blockSize - 1) // blockSize is a power of two
-	tr.curr = &regFileReader{r: tr.r, nb: nb}
-
-	// Check for old GNU sparse format entry.
-	if hdr.Typeflag == TypeGNUSparse {
-		// Get the real size of the file.
-		hdr.Size = p.parseNumeric(tr.blk.GNU().RealSize())
-		if p.err != nil {
-			tr.err = p.err
-			return nil
-		}
-
-		// Read the sparse map.
-		sp := tr.readOldGNUSparseMap(&tr.blk)
-		if tr.err != nil {
-			return nil
-		}
-
-		// Current file is a GNU sparse file. Update the current file reader.
-		tr.curr, tr.err = newSparseFileReader(tr.curr, sp, hdr.Size)
-		if tr.err != nil {
-			return nil
-		}
-	}
-
+	// Check for parsing errors.
 	if p.err != nil {
 		tr.err = p.err
-		return nil
+		return nil, nil
 	}
-
-	return hdr
+	return hdr, &tr.blk
 }
 
 // readOldGNUSparseMap reads the sparse map as stored in the old GNU sparse format.