libavutil: Add wchartoutf8(), wchartoansi(), utf8toansi(), getenv_utf8(), freeenv_utf8() and getenv_dup()

wchartoutf8() converts strings returned by WinAPI into UTF-8,
which is FFmpeg's preffered encoding.

Some external dependencies, such as AviSynth, are still
not Unicode-enabled. utf8toansi() converts UTF-8 strings
into ANSI in two steps: UTF-8 -> wchar_t -> ANSI.
wchartoansi() is responsible for the second step of the conversion.
Conversion in just one step is not supported by WinAPI.

Since these character converting functions allocate the buffer
of necessary size, they also facilitate the removal of MAX_PATH limit
in places where fixed-size ANSI/WCHAR strings were used
as filename buffers.

On Windows, getenv_utf8() wraps _wgetenv() converting its input from
and its output to UTF-8. Strings returned by getenv_utf8()
must be freed by freeenv_utf8().

On all other platforms getenv_utf8() is a wrapper around getenv(),
and freeenv_utf8() is a no-op.

The value returned by plain getenv() cannot be modified;
av_strdup() is usually used when modifications are required.
However, on Windows, av_strdup() after getenv_utf8() leads to
unnecessary allocation. getenv_dup() is introduced to avoid
such an allocation. Value returned by getenv_dup() must be freed
by av_free().

Because of cleanup complexities, in places that only test the existence
of an environment variable or compare its value with a string
consisting entirely of ASCII characters, the use of plain getenv()
is still preferred. (libavutil/log.c check_color_terminal()
is an example of such a place.)

Plain getenv() is also preffered in UNIX-only code,
such as bktr.c, fbdev_common.c, oss.c in libavdevice
or af_ladspa.c in libavfilter.

Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -2272,6 +2272,7 @@ SYSTEM_FUNCS="
     fcntl
     getaddrinfo
     getauxval
+    getenv
     gethrtime
     getopt
     GetModuleHandle

libavutil/getenv_utf8.h

@@ -0,0 +1,86 @@
+/*
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#ifndef AVUTIL_GETENV_UTF8_H
+#define AVUTIL_GETENV_UTF8_H
+
+#include <stdlib.h>
+
+#include "config.h"
+#include "mem.h"
+
+#if HAVE_GETENV && defined(_WIN32)
+
+#include "libavutil/wchar_filename.h"
+
+static inline char *getenv_utf8(const char *varname)
+{
+    wchar_t *varname_w, *var_w;
+    char *var;
+
+    if (utf8towchar(varname, &varname_w))
+        return NULL;
+    if (!varname_w)
+        return NULL;
+
+    var_w = _wgetenv(varname_w);
+    av_free(varname_w);
+
+    if (!var_w)
+        return NULL;
+    if (wchartoutf8(var_w, &var))
+        return NULL;
+
+    return var;
+
+    // No CP_ACP fallback compared to other *_utf8() functions:
+    // non UTF-8 strings must not be returned.
+}
+
+static inline void freeenv_utf8(char *var)
+{
+    av_free(var);
+}
+
+static inline char *getenv_dup(const char *varname)
+{
+    return getenv_utf8(varname);
+}
+
+#else
+
+static inline char *getenv_utf8(const char *varname)
+{
+    return getenv(varname);
+}
+
+static inline void freeenv_utf8(char *var)
+{
+}
+
+static inline char *getenv_dup(const char *varname)
+{
+    char *var = getenv(varname);
+    if (!var)
+        return NULL;
+    return av_strdup(var);
+}
+
+#endif // HAVE_GETENV && defined(_WIN32)
+
+#endif // AVUTIL_GETENV_UTF8_H

libavutil/wchar_filename.h

@@ -20,6 +20,8 @@
 #define AVUTIL_WCHAR_FILENAME_H
 
 #ifdef _WIN32
+
+#define WIN32_LEAN_AND_MEAN
 #include <windows.h>
 #include "mem.h"
 
@@ -41,6 +43,57 @@ static inline int utf8towchar(const char *filename_utf8, wchar_t **filename_w)
     return 0;
 }
 
+av_warn_unused_result
+static inline int wchartocp(unsigned int code_page, const wchar_t *filename_w,
+                            char **filename)
+{
+    DWORD flags = code_page == CP_UTF8 ? WC_ERR_INVALID_CHARS : 0;
+    int num_chars = WideCharToMultiByte(code_page, flags, filename_w, -1,
+                                        NULL, 0, NULL, NULL);
+    if (num_chars <= 0) {
+        *filename = NULL;
+        return 0;
+    }
+    *filename = av_malloc_array(num_chars, sizeof *filename);
+    if (!*filename) {
+        errno = ENOMEM;
+        return -1;
+    }
+    WideCharToMultiByte(code_page, flags, filename_w, -1,
+                        *filename, num_chars, NULL, NULL);
+    return 0;
+}
+
+av_warn_unused_result
+static inline int wchartoutf8(const wchar_t *filename_w, char **filename)
+{
+    return wchartocp(CP_UTF8, filename_w, filename);
+}
+
+av_warn_unused_result
+static inline int wchartoansi(const wchar_t *filename_w, char **filename)
+{
+    return wchartocp(CP_ACP, filename_w, filename);
+}
+
+av_warn_unused_result
+static inline int utf8toansi(const char *filename_utf8, char **filename)
+{
+    wchar_t *filename_w = NULL;
+    int ret = -1;
+    if (utf8towchar(filename_utf8, &filename_w))
+        return -1;
+
+    if (!filename_w) {
+        *filename = NULL;
+        return 0;
+    }
+
+    ret = wchartoansi(filename_w, filename);
+    av_free(filename_w);
+    return ret;
+}
+
 /**
  * Checks for extended path prefixes for which normalization needs to be skipped.
  * see .NET6: PathInternal.IsExtended()