[PATCH] erofs-utils: mkfs: fix CPU spin using --tar=f when stdin is closed

[David Scott <dave@recoil.org>]

On my Mac I saw a CPU spin which looked like this:
```
Call graph:
    2192 Thread_132504   DispatchQueue_1: com.apple.main-thread  (serial)
      2192 start  (in dyld) + 6992  [0x185bcbda4]
        2192 main  (in mkfs.erofs) + 7916  [0x10253a6d0]
          2192 tarerofs_parse_tar  (in mkfs.erofs) + 5492  [0x102551d48]
            2187 tarerofs_write_file_data  (in mkfs.erofs) + 140  [0x102551fe0]
            + 2187 write  (in libsystem_kernel.dylib) + 8  [0x185f47834]
            4 tarerofs_write_file_data  (in mkfs.erofs) + 116  [0x102551fc8]
            + 4 erofs_iostream_read  (in mkfs.erofs) + 16,36,...  [0x10254fa28,0x10254fa3c,...]
            1 tarerofs_write_file_data  (in mkfs.erofs) + 140  [0x102551fe0]
```

The input stream was closed prematurely, so the reads returned 0 (EOF),
which wasn't considered an error.

Treat return of 0 (EOF) as an error.

Reproduce by:
```
dd if=/dev/zero bs=1024 count=4 2>/dev/null > /tmp/testfile
COPYFILE_DISABLE=1 tar cf - -C /tmp testfile | head -c 2048 > /tmp/truncated.tar
./mkfs/mkfs.erofs --tar=f output.erofs < /tmp/truncated.tar
```
Before the patch this will hang, after it should fail as expected.

(COPYFILE_DISABLE tells mac to avoid putting extra stuff in the tar)

Closes: https://github.com/erofs/erofs-utils/issues/43
Signed-off-by: David Scott <dave@recoil.org>
---
 lib/tar.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/lib/tar.c b/lib/tar.c
index 178f843..57c6fee 100644
--- a/lib/tar.c
+++ b/lib/tar.c
@@ -638,8 +638,11 @@ static int tarerofs_write_uncompressed_file(struct erofs_inode *inode,
 
 	for (pos = 0; pos < inode->i_size; pos += ret) {
 		ret = erofs_iostream_read(&tar->ios, &buf, inode->i_size - pos);
-		if (ret < 0)
+		if (ret <= 0) {
+			if (!ret)
+				ret = -EIO;
 			break;
+		}
 		if (erofs_dev_write(sbi, buf,
 				    erofs_pos(sbi, inode->u.i_blkaddr) + pos,
 				    ret)) {
@@ -649,6 +652,8 @@ static int tarerofs_write_uncompressed_file(struct erofs_inode *inode,
 	}
 	inode->idata_size = 0;
 	inode->datasource = EROFS_INODE_DATA_SOURCE_NONE;
+	if (ret < 0)
+		return ret;
 	return 0;
 }
 
@@ -673,8 +678,11 @@ static int tarerofs_write_file_data(struct erofs_inode *inode,
 
 	for (j = inode->i_size; j; ) {
 		nread = erofs_iostream_read(&tar->ios, &buf, j);
-		if (nread < 0)
+		if (nread <= 0) {
+			if (!nread)
+				nread = -EIO;
 			break;
+		}
 		if (pwrite(fd, buf, nread, off) != nread) {
 			nread = -EIO;
 			break;
@@ -684,6 +692,8 @@ static int tarerofs_write_file_data(struct erofs_inode *inode,
 	}
 	erofs_diskbuf_commit(inode->i_diskbuf, inode->i_size);
 	inode->datasource = EROFS_INODE_DATA_SOURCE_DISKBUF;
+	if (nread < 0)
+		return nread;
 	return 0;
 }
 
-- 
2.43.0