Perform better fixups of NT paths in execve()

This change ensures we do a better job translating /c/foo.bar paths into
c:/foo.bar paths on Windows when generating the CreateProcess() cmd line
thus fixing a regression that happened in the last two months when using
the help() feature of Actually Portable Python in the CMD.EXE shell.

libc/calls/mkntcmdline.c

@@ -41,6 +41,10 @@ static bool NeedsQuotes(const char *s) {
   return false;
 }
 
+static inline int IsAlpha(int c) {
+  return ('A' <= c && c <= 'Z') || ('a' <= c && c <= 'z');
+}
+
 /**
  * Converts System V argv to Windows-style command line.
  *
@@ -86,8 +90,27 @@ textwindows int mkntcmdline(char16_t cmdline[ARG_MAX / 2], const char *prog,
         }
       }
       if (!x) break;
-      if (!i && x == '/') {
+      if (x == '/' || x == '\\') {
-        x = '\\';
+        if (!i) {
+          // turn / into \ for first arg
+          x = '\\';
+          // turn \c\... into c:\ for first arg
+          if (k == 2 && IsAlpha(cmdline[1]) && cmdline[0] == '\\') {
+            cmdline[0] = cmdline[1];
+            cmdline[1] = ':';
+          }
+        } else {
+          // turn stuff like `less /c/...`
+          //            into `less c:/...`
+          // turn stuff like `more <\\\"/c/...\\\"`
+          //            into `more <\\\"c:/...\\\"`
+          if (k > 3 && IsAlpha(cmdline[k - 1]) &&
+              (cmdline[k - 2] == '/' || cmdline[k - 2] == '\\') &&
+              (cmdline[k - 3] == '"' || cmdline[k - 3] == ' ')) {
+            cmdline[k - 2] = cmdline[k - 1];
+            cmdline[k - 1] = ':';
+          }
+        }
       }
       if (x == '\\') {
         ++slashes;

libc/calls/mkntenvblock.c

@@ -72,12 +72,14 @@ static textwindows void FixPath(char *path) {
 
   // turn \c\... into c:\...
   p = path;
-  if (p[0] == '/' && IsAlpha(p[1]) && p[2] == '/') {
+  if ((p[0] == '/' | p[0] == '\\') && IsAlpha(p[1]) &&
+      (p[2] == '/' || p[2] == '\\')) {
     p[0] = p[1];
     p[1] = ':';
   }
   for (; *p; ++p) {
-    if (p[0] == ';' && p[1] == '/' && IsAlpha(p[2]) && p[3] == '/') {
+    if (p[0] == ';' && (p[1] == '/' || p[1] == '\\') && IsAlpha(p[2]) &&
+        (p[3] == '/' || p[3] == '\\')) {
       p[1] = p[2];
       p[2] = ':';
     }

libc/intrin/describestat.greg.c

@@ -79,8 +79,10 @@ const char *DescribeStat(int rc, const struct stat *st) {
     i += ksnprintf(buf + i, n - i, ", .st_%s=%'lu", "blksize", st->st_blksize);
   }
 
-  buf[i++] = '}';
+  if (n - i >= 2) {
-  buf[i] = 0;
+    buf[i + 0] = '}';
+    buf[i + 1] = 0;
+  }
 
   return buf;
 }

test/libc/calls/mkntcmdline_test.c

@@ -78,16 +78,29 @@ TEST(mkntcmdline, testUnicode) {
                cmdline);
 }
 
-TEST(mkntcmdline, fix) {
+TEST(mkntcmdline, fixAsBestAsWeCanForNow1) {
   char *argv1[] = {
-      "C:/WINDOWS/system32/cmd.exe",
+      "/C/WINDOWS/system32/cmd.exe",
       "/C",
-      "more < \"C:\\Users\\jart\\AppData\\Local\\Temp\\tmplquaa_d6\"",
+      "more < \"/C/Users/jart/AppData/Local/Temp/tmplquaa_d6\"",
       NULL,
   };
   EXPECT_NE(-1, mkntcmdline(cmdline, argv1[0], argv1));
   EXPECT_STREQ(u"C:\\WINDOWS\\system32\\cmd.exe /C \"more < "
-               u"\\\"C:\\Users\\jart\\AppData\\Local\\Temp\\tmplquaa_d6\\\"\"",
+               u"\\\"C:/Users/jart/AppData/Local/Temp/tmplquaa_d6\\\"\"",
+               cmdline);
+}
+
+TEST(mkntcmdline, fixAsBestAsWeCanForNow2) {
+  char *argv1[] = {
+      "/C/WINDOWS/system32/cmd.exe",
+      "/C",
+      "less /C/Users/jart/AppData/Local/Temp/tmplquaa_d6",
+      NULL,
+  };
+  EXPECT_NE(-1, mkntcmdline(cmdline, argv1[0], argv1));
+  EXPECT_STREQ(u"C:\\WINDOWS\\system32\\cmd.exe /C \"less "
+               u"C:/Users/jart/AppData/Local/Temp/tmplquaa_d6\"",
                cmdline);
 }