Add utility to serialize windows arguments

Add a function to convert an argv array into a single escaped string to
be passed to CreateProcess() on Windows.

Refs <https://learn.microsoft.com/en-us/cpp/c-language/parsing-c-command-line-arguments>
Refs <https://learn.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-createprocessw>

app/meson.build

@@ -75,6 +75,7 @@ conf.set('_GNU_SOURCE', true)
 if host_machine.system() == 'windows'
     windows = import('windows')
     src += [
+        'src/util/command.c',
         'src/sys/win/file.c',
         'src/sys/win/process.c',
         windows.compile_resources('scrcpy-windows.rc'),
@@ -238,6 +239,12 @@ if get_option('buildtype') == 'debug'
             'src/util/strbuf.c',
             'src/util/term.c',
         ]],
+        ['test_command_windows', [
+          'tests/test_command_windows.c',
+          'src/util/command.c',
+          'src/util/str.c',
+          'src/util/strbuf.c',
+        ]],
         ['test_control_msg_serialize', [
             'tests/test_control_msg_serialize.c',
             'src/control_msg.c',

app/src/sys/win/process.c

@@ -3,9 +3,10 @@
 #include <processthreadsapi.h>
 
 #include <assert.h>
+#include <stdlib.h>
 
 #include "util/log.h"
-#include "util/str.h"
+#include "util/strbuf.h"
 
 #define CMD_MAX_LEN 8192
 

app/src/util/command.c

@@ -0,0 +1,99 @@
+#include "command.h"
+
+#include <stdlib.h>
+
+#include "util/strbuf.h"
+
+char *
+sc_command_serialize_windows(const char *const argv[]) {
+    // <https://learn.microsoft.com/en-us/cpp/c-language/parsing-c-command-line-arguments>
+    // <https://learn.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-createprocessw>
+
+    struct sc_strbuf buf;
+    bool ok = sc_strbuf_init(&buf, 1024);
+    if (!ok) {
+        return NULL;
+    }
+
+#define BUF_PUSH(C) \
+    do { \
+        ok = sc_strbuf_append_char(&buf, C); \
+        if (!ok) { \
+            goto end; \
+        } \
+    } while (0)
+
+    while (*argv) {
+        const char *arg = *argv;
+
+        BUF_PUSH('"');
+
+        // <https://learn.microsoft.com/en-us/cpp/c-language/parsing-c-command-line-arguments>
+        //
+        // """
+        // If an even number of backslashes is followed by a double quote mark,
+        // then one backslash (\) is placed in the argv array for every pair of
+        // backslashes (\\), and the double quote mark (") is interpreted as a
+        // string delimiter.
+        //
+        // If an odd number of backslashes is followed by a double quote mark,
+        // then one backslash (\) is placed in the argv array for every pair of
+        // backslashes (\\). The double quote mark is interpreted as an escape
+        // sequence by the remaining backslash, causing a literal double quote
+        // mark (") to be placed in argv.
+        // """
+        //
+        // To produce correct escaping according to what the parser will do, we
+        // must count the number of successive backslashes.
+        unsigned backslashes = 0;
+
+        for (const char *c = arg; *c; c++) {
+            switch (*c) {
+                case '"':
+                    while (backslashes) {
+                        // Double all backslashes before a quote
+                        BUF_PUSH('\\');
+                        BUF_PUSH('\\');
+                        --backslashes;
+                    }
+                    BUF_PUSH('\\');
+                    BUF_PUSH('"');
+                    backslashes = 0;
+                    break;
+                case '\\':
+                    ++backslashes;
+                    break;
+                default:
+                    while (backslashes) {
+                        // Put all backslashes as literals
+                        BUF_PUSH('\\');
+                        --backslashes;
+                    }
+                    BUF_PUSH(*c);
+                    break;
+            }
+        }
+
+        while (backslashes) {
+            // Double all backslashes before a quote
+            BUF_PUSH('\\');
+            BUF_PUSH('\\');
+            --backslashes;
+        }
+
+        BUF_PUSH('"');
+
+        ++argv;
+
+        // Argument separator
+        if (*argv) {
+            BUF_PUSH(' ');
+        }
+    }
+
+    return buf.s;
+
+end:
+    free(buf.s);
+    return NULL;
+}

app/src/util/command.h

@@ -0,0 +1,17 @@
+#ifndef SC_COMMAND_H
+#define SC_COMMAND_H
+
+#include "common.h"
+
+/**
+ * Serialize an argv array for Windows
+ *
+ * Convert a NULL-terminated argument array into a single escaped string
+ * suitable for massing to CreateProcess() on Windows.
+ *
+ * The returned value must be freed by the caller.
+ */
+char *
+sc_command_serialize_windows(const char *const argv[]);
+
+#endif

app/tests/test_command_windows.c

@@ -0,0 +1,61 @@
+#include "common.h"
+
+#include <assert.h>
+#include <stdlib.h>
+
+#include "util/command.h"
+
+static void test_command_with_spaces(void) {
+    const char *const argv[] = {
+        "C:\\Program Files\\scrcpy\\adb",
+        "-s",
+        "serial with spaces",
+        "push",
+        "E:\\some folder\\scrcpy-server",
+        "/data/local/tmp/scrcpy-server.jar",
+        NULL,
+    };
+    char *cmd = sc_command_serialize_windows(argv);
+    const char *expected = "\"C:\\Program Files\\scrcpy\\adb\" "
+                           "\"-s\" "
+                           "\"serial with spaces\" "
+                           "\"push\" "
+                           "\"E:\\some folder\\scrcpy-server\" "
+                           "\"/data/local/tmp/scrcpy-server.jar\"";
+
+    assert(!strcmp(expected, cmd));
+    free(cmd);
+}
+
+static void test_command_with_backslashes(void) {
+    const char *const argv[] = {
+        "a\\\\ b\\",
+        "def \\",
+        "gh\"i\" \\\\",
+        "jkl\\\\",
+        "mno\\",
+        "p\\\"qr",
+        NULL,
+    };
+
+    char *cmd = sc_command_serialize_windows(argv);
+    const char *expected = "\"a\\\\ b\\\\\" "
+                           "\"def \\\\\" "
+                           "\"gh\\\"i\\\" \\\\\\\\\" "
+                           "\"jkl\\\\\\\\\" "
+                           "\"mno\\\\\" "
+                           "\"p\\\\\\\"qr\"";
+
+    assert(!strcmp(expected, cmd));
+    free(cmd);
+}
+
+int main(int argc, char *argv[]) {
+    (void) argc;
+    (void) argv;
+
+    test_command_with_spaces();
+    test_command_with_backslashes();
+
+    return 0;
+}