xz/DOS: Add experimental 8.3 filename support.

This is incompatible with the 8.3 support patch made by
Juan Manuel Guerrero. I think this one is nicer, but
I need to get feedback from DOS users before saying
that this is the final version of 8.3 filename support.

src/xz/suffix.c

@@ -12,6 +12,10 @@
 
 #include "private.h"
 
+#ifdef __DJGPP__
+#	include <fcntl.h>
+#endif
+
 // For case-insensitive filename suffix on case-insensitive systems
 #if defined(TUKLIB_DOSLIKE) || defined(__VMS)
 #	define strcmp strcasecmp
@@ -45,6 +49,31 @@ has_dir_sep(const char *str)
 }
 
 
+#ifdef __DJGPP__
+/// \brief      Test for special suffix used for 8.3 short filenames (SFN)
+///
+/// \return     If str matches *.?- or *.??-, true is returned. Otherwise
+///             false is returned.
+static bool
+has_sfn_suffix(const char *str, size_t len)
+{
+	if (len >= 4 && str[len - 1] == '-' && str[len - 2] != '.'
+			&& !is_dir_sep(str[len - 2])) {
+		// *.?-
+		if (str[len - 3] == '.')
+			return !is_dir_sep(str[len - 4]);
+
+		// *.??-
+		if (len >= 5 && !is_dir_sep(str[len - 3])
+				&& str[len - 4] == '.')
+			return !is_dir_sep(str[len - 5]);
+	}
+
+	return false;
+}
+#endif
+
+
 /// \brief      Checks if src_name has given compressed_suffix
 ///
 /// \param      suffix      Filename suffix to look for
@@ -87,6 +116,9 @@ uncompressed_name(const char *src_name, const size_t src_len)
 		{ ".xz",    "" },
 		{ ".txz",   ".tar" }, // .txz abbreviation for .txt.gz is rare.
 		{ ".lzma",  "" },
+#ifdef __DJGPP__
+		{ ".lzm",   "" },
+#endif
 		{ ".tlz",   ".tar" },
 		// { ".gz",    "" },
 		// { ".tgz",   ".tar" },
@@ -112,6 +144,17 @@ uncompressed_name(const char *src_name, const size_t src_len)
 				break;
 			}
 		}
+
+#ifdef __DJGPP__
+		// Support also *.?- -> *.? and *.??- -> *.?? on DOS.
+		// This is done also when long filenames are available
+		// to keep it easy to decompress files created when
+		// long filename support wasn't available.
+		if (new_len == 0 && has_sfn_suffix(src_name, src_len)) {
+			new_suffix = "";
+			new_len = src_len - 1;
+		}
+#endif
 	}
 
 	if (new_len == 0 && custom_suffix != NULL)
@@ -134,21 +177,35 @@ uncompressed_name(const char *src_name, const size_t src_len)
 }
 
 
+/// This message is needed in multiple places in compressed_name(),
+/// so the message has been put into its own function.
+static void
+msg_suffix(const char *src_name, const char *suffix)
+{
+	message_warning(_("%s: File already has `%s' suffix, skipping"),
+			src_name, suffix);
+	return;
+}
+
+
 /// \brief      Appends suffix to src_name
 ///
 /// In contrast to uncompressed_name(), we check only suffixes that are valid
 /// for the specified file format.
 static char *
-compressed_name(const char *src_name, const size_t src_len)
+compressed_name(const char *src_name, size_t src_len)
 {
 	// The order of these must match the order in args.h.
-	static const char *const all_suffixes[][3] = {
+	static const char *const all_suffixes[][4] = {
 		{
 			".xz",
 			".txz",
 			NULL
 		}, {
 			".lzma",
+#ifdef __DJGPP__
+			".lzm",
+#endif
 			".tlz",
 			NULL
 /*
@@ -170,20 +227,27 @@ compressed_name(const char *src_name, const size_t src_len)
 	const size_t format = opt_format - 1;
 	const char *const *suffixes = all_suffixes[format];
 
+	// Look for known filename suffixes and refuse to compress them.
 	for (size_t i = 0; suffixes[i] != NULL; ++i) {
 		if (test_suffix(suffixes[i], src_name, src_len) != 0) {
-			message_warning(_("%s: File already has `%s' "
+			msg_suffix(src_name, suffixes[i]);
-					"suffix, skipping"), src_name,
-					suffixes[i]);
 			return NULL;
 		}
 	}
 
+#ifdef __DJGPP__
+	// Recognize also the special suffix that is used when long
+	// filename (LFN) support isn't available. This suffix is
+	// recognized on LFN systems too.
+	if (opt_format == FORMAT_XZ && has_sfn_suffix(src_name, src_len)) {
+		msg_suffix(src_name, "-");
+		return NULL;
+	}
+#endif
+
 	if (custom_suffix != NULL) {
 		if (test_suffix(custom_suffix, src_name, src_len) != 0) {
-			message_warning(_("%s: File already has `%s' "
+			msg_suffix(src_name, custom_suffix);
-					"suffix, skipping"), src_name,
-					custom_suffix);
 			return NULL;
 		}
 	}
@@ -199,7 +263,101 @@ compressed_name(const char *src_name, const size_t src_len)
 
 	const char *suffix = custom_suffix != NULL
 			? custom_suffix : suffixes[0];
-	const size_t suffix_len = strlen(suffix);
+	size_t suffix_len = strlen(suffix);
+
+#ifdef __DJGPP__
+	if (!_use_lfn(src_name)) {
+		// Long filename (LFN) support isn't available and we are
+		// limited to 8.3 short filenames (SFN).
+		//
+		// Look for suffix separator from the filename, and make sure
+		// that it is in the filename, not in a directory name.
+		const char *sufsep = strrchr(src_name, '.');
+		if (sufsep == NULL || sufsep[1] == '\0'
+				|| has_dir_sep(sufsep)) {
+			// src_name has no filename extension.
+			//
+			// Examples:
+			// xz foo         -> foo.xz
+			// xz -F lzma foo -> foo.lzm
+			// xz -S x foo    -> foox
+			// xz -S x foo.   -> foo.x
+			// xz -S x.y foo  -> foox.y
+			// xz -S .x foo   -> foo.x
+			// xz -S .x foo.  -> foo.x
+			//
+			// Avoid double dots:
+			if (sufsep != NULL && sufsep[1] == '\0'
+					&& suffix[0] == '.')
+				--src_len;
+
+		} else if (custom_suffix == NULL
+				&& strcasecmp(sufsep, ".tar") == 0) {
+			// ".tar" is handled specially.
+			//
+			// Examples:
+			// xz foo.tar          -> foo.txz
+			// xz -F lzma foo.tar  -> foo.tlz
+			static const char *const tar_suffixes[] = {
+				".txz",
+				".tlz",
+				// ".tgz",
+			};
+			suffix = tar_suffixes[format];
+			suffix_len = 4;
+			src_len -= 4;
+
+		} else {
+			if (custom_suffix == NULL && opt_format == FORMAT_XZ) {
+				// Instead of the .xz suffix, use a single
+				// character at the end of the filename
+				// extension. This is to minimize name
+				// conflicts when compressing multiple files
+				// with the same basename. E.g. foo.txt and
+				// foo.exe become foo.tx- and foo.ex-. Dash
+				// is rare as the last character of the
+				// filename extension, so it seems to be
+				// quite safe choice and it stands out better
+				// in directory listings than e.g. x. For
+				// comparison, gzip uses z.
+				suffix = "-";
+				suffix_len = 1;
+			}
+
+			if (suffix[0] == '.') {
+				// The first character of the suffix is a dot.
+				// Throw away the original filename extension
+				// and replace it with the new suffix.
+				//
+				// Examples:
+				// xz -F lzma foo.txt  -> foo.lzm
+				// xz -S .x  foo.txt   -> foo.x
+				src_len = sufsep - src_name;
+
+			} else {
+				// The first character of the suffix is not
+				// a dot. Preserve the first 0-2 characters
+				// of the original filename extension.
+				//
+				// Examples:
+				// xz foo.txt         -> foo.tx-
+				// xz -S x  foo.c     -> foo.cx
+				// xz -S ab foo.c     -> foo.cab
+				// xz -S ab foo.txt   -> foo.tab
+				// xz -S abc foo.txt  -> foo.abc
+				//
+				// Truncate the suffix to three chars:
+				if (suffix_len > 3)
+					suffix_len = 3;
+
+				// If needed, overwrite 1-3 characters.
+				if (strlen(sufsep) > 4 - suffix_len)
+					src_len = sufsep - src_name
+							+ 4 - suffix_len;
+			}
+		}
+	}
+#endif
 
 	char *dest_name = xmalloc(src_len + suffix_len + 1);