hex-escape: (en|de)code strings to/from restricted character set

The character set is chosen to be suitable for pathnames, and the same
as that used by contrib/nmbug

[With additions by Jani Nikula]

util/Makefile.local

@@ -3,7 +3,7 @@
 dir := util
 extra_cflags += -I$(srcdir)/$(dir)
 
-libutil_c_srcs := $(dir)/xutil.c $(dir)/error_util.c
+libutil_c_srcs := $(dir)/xutil.c $(dir)/error_util.c $(dir)/hex-escape.c
 
 libutil_modules := $(libutil_c_srcs:.c=.o)
 

util/hex-escape.c

@@ -0,0 +1,161 @@
+/* hex-escape.c -  Manage encoding and decoding of byte strings into path names
+ *
+ * Copyright (c) 2011 David Bremner
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see http://www.gnu.org/licenses/ .
+ *
+ * Author: David Bremner <david@tethera.net>
+ */
+
+#include <assert.h>
+#include <string.h>
+#include <talloc.h>
+#include <ctype.h>
+#include "error_util.h"
+#include "hex-escape.h"
+
+static const size_t default_buf_size = 1024;
+
+static const char *output_charset =
+    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+-_@=.,";
+
+static const char escape_char = '%';
+
+static int
+is_output (char c)
+{
+    return (strchr (output_charset, c) != NULL);
+}
+
+static int
+maybe_realloc (void *ctx, size_t needed, char **out, size_t *out_size)
+{
+    if (*out_size < needed) {
+
+	if (*out == NULL)
+	    *out = talloc_size (ctx, needed);
+	else
+	    *out = talloc_realloc (ctx, *out, char, needed);
+
+	if (*out == NULL)
+	    return 0;
+
+	*out_size = needed;
+    }
+    return 1;
+}
+
+hex_status_t
+hex_encode (void *ctx, const char *in, char **out, size_t *out_size)
+{
+
+    const char *p;
+    char *q;
+
+    size_t needed = 1;  /* for the NUL */
+
+    assert (ctx); assert (in); assert (out); assert (out_size);
+
+    for (p = in; *p; p++) {
+	needed += is_output (*p) ? 1 : 3;
+    }
+
+    if (*out == NULL)
+	*out_size = 0;
+
+    if (!maybe_realloc (ctx, needed, out, out_size))
+	return HEX_OUT_OF_MEMORY;
+
+    q = *out;
+    p = in;
+
+    while (*p) {
+	if (is_output (*p)) {
+	    *q++ = *p++;
+	} else {
+	    sprintf (q, "%%%02x", (unsigned char)*p++);
+	    q += 3;
+	}
+    }
+
+    *q = '\0';
+    return HEX_SUCCESS;
+}
+
+/* Hex decode 'in' to 'out'.
+ *
+ * This must succeed for in == out to support hex_decode_inplace().
+ */
+static hex_status_t
+hex_decode_internal (const char *in, unsigned char *out)
+{
+    char buf[3];
+
+    while (*in) {
+	if (*in == escape_char) {
+	    char *endp;
+
+	    /* This also handles unexpected end-of-string. */
+	    if (!isxdigit ((unsigned char) in[1]) ||
+		!isxdigit ((unsigned char) in[2]))
+		return HEX_SYNTAX_ERROR;
+
+	    buf[0] = in[1];
+	    buf[1] = in[2];
+	    buf[2] = '\0';
+
+	    *out = strtoul (buf, &endp, 16);
+
+	    if (endp != buf + 2)
+		return HEX_SYNTAX_ERROR;
+
+	    in += 3;
+	    out++;
+	} else {
+	    *out++ = *in++;
+	}
+    }
+
+    *out = '\0';
+
+    return HEX_SUCCESS;
+}
+
+hex_status_t
+hex_decode_inplace (char *s)
+{
+    /* A decoded string is never longer than the encoded one, so it is
+     * safe to decode a string onto itself. */
+    return hex_decode_internal (s, (unsigned char *) s);
+}
+
+hex_status_t
+hex_decode (void *ctx, const char *in, char **out, size_t * out_size)
+{
+    const char *p;
+    size_t needed = 1;	/* for the NUL */
+
+    assert (ctx); assert (in); assert (out); assert (out_size);
+
+    for (p = in; *p; p++)
+	if ((p[0] == escape_char) && isxdigit (p[1]) && isxdigit (p[2]))
+	    needed -= 1;
+	else
+	    needed += 1;
+
+    if (!maybe_realloc (ctx, needed, out, out_size))
+	return HEX_OUT_OF_MEMORY;
+
+    return hex_decode_internal (in, (unsigned char *) *out);
+}

util/hex-escape.h

@@ -0,0 +1,41 @@
+#ifndef _HEX_ESCAPE_H
+#define _HEX_ESCAPE_H
+
+typedef enum hex_status {
+    HEX_SUCCESS = 0,
+    HEX_SYNTAX_ERROR,
+    HEX_OUT_OF_MEMORY
+} hex_status_t;
+
+/*
+ * The API for hex_encode() and hex_decode() is modelled on that for
+ * getline.
+ *
+ * If 'out' points to a NULL pointer a char array of the appropriate
+ * size is allocated using talloc, and out_size is updated.
+ *
+ * If 'out' points to a non-NULL pointer, it assumed to describe an
+ * existing char array, with the size given in *out_size.  This array
+ * may be resized by talloc_realloc if needed; in this case *out_size
+ * will also be updated.
+ *
+ * Note that it is an error to pass a NULL pointer for any parameter
+ * of these routines.
+ */
+
+hex_status_t
+hex_encode (void *talloc_ctx, const char *in, char **out,
+            size_t *out_size);
+
+hex_status_t
+hex_decode (void *talloc_ctx, const char *in, char **out,
+            size_t *out_size);
+
+/*
+ * Non-allocating hex decode to decode 's' in-place. The length of the
+ * result is always equal to or shorter than the length of the
+ * original.
+ */
+hex_status_t
+hex_decode_inplace (char *s);
+#endif