Improve zip read-only filesystem

readdir() will now always yield an inode that's consistent with stat()
on ZipOS and Windows in general. More APIs have been updated to return
the appropriate error code when inappropriately trying to do ops, like
sockets, with a zip file descriptor. The path normalization algorithms
are now fully fleshed out. Some socket APIs have been fixed so they'll
raise EBADF vs. ENOTSOCK appropriately. Lastly seekdir() will now work
properly on NetBSD and FreeBSD (not sure why anyone would even use it)

libc/runtime/zipos-normpath.c

@@ -18,63 +18,45 @@
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/runtime/zipos.internal.h"
 
-static size_t __zipos_trimpath(char *s, int *isabs) {
-  char *p = s, *q = s;
-  for (; *q; ++q) {
-    if (*q == '/') {
-      while (q[1] == '/') ++q;
-      if (q[1] == '.' && (q[2] == '/' || q[2] == '\0')) {
-        ++q;
-      } else {
-        *p++ = '/';
-      }
+// normalizes zip filesystem path w/ overlapping strlcpy() style api
+// zip paths look like relative paths, but they're actually absolute
+// with respect to the archive; so similar to how /../etc would mean
+// /etc, we'd translate that here to "etc". when storing assets in a
+// zip archive, callers should append trailing slash for directories
+// returns strlen of 𝑑; returns 𝑛 when insufficient buffer available
+// nul terminator is guaranteed if n>0. it's fine if 𝑑 and 𝑠 overlap
+// test vectors for this algorithm in: test/libc/stdio/zipdir_test.c
+size_t __zipos_normpath(char *d, const char *s, size_t n) {
+  char *p, *e;
+  for (p = d, e = d + n; p < e && *s; ++s) {
+    if ((p == d || p[-1] == '/') && *s == '/') {
+      // matched "^/" or "//"
+    } else if ((p == d || p[-1] == '/') &&  //
+               s[0] == '.' &&               //
+               (!s[1] || s[1] == '/')) {
+      // matched "/./" or "^.$" or "^./" or "/.$"
+      s += !!s[1];
+    } else if ((p == d || p[-1] == '/') &&  //
+               s[0] == '.' &&               //
+               s[1] == '.' &&               //
+               (!s[2] || s[2] == '/')) {
+      // matched "/../" or "^..$" or "^../" or "/..$"
+      while (p > d && p[-1] == '/') --p;
+      while (p > d && p[-1] != '/') --p;
     } else {
-      *p++ = *q;
+      *p++ = *s;
     }
   }
-  if (s < p && p[-1] == '.' && p[-2] == '.' && (p - 2 == s || p[-3] == '/')) {
-    *p++ = '/';
+  // if we didn't overflow
+  if (p < e) {
+    // trim trailing slashes and add nul terminator
+    while (p > d && p[-1] == '/') --p;
+    *p = '\0';
+  } else {
+    // force nul-terminator to exist if possible
+    if (p > d) {
+      p[-1] = '\0';
+    }
   }
-  *p = '\0';
-  if (isabs) {
-    *isabs = *s == '/';
-  }
-  return p - s;
-}
-
-size_t __zipos_normpath(char *s) {
-  int isabs;
-  char *p = s, *q = s;
-  __zipos_trimpath(s, &isabs);
-  if (!*s) return 0;
-  for (; *q != '\0'; ++q) {
-    if (q[0] == '/' && q[1] == '.' && q[2] == '.' &&
-        (q[3] == '/' || q[3] == '\0')) {
-      char *ep = p;
-      while (s < ep && *--ep != '/') donothing;
-      if (ep != p &&
-          (p[-1] != '.' || p[-2] != '.' || (s < p - 3 && p[-3] != '/'))) {
-        p = ep;
-        q += 2;
-        continue;
-      } else if (ep == s && isabs) {
-        q += 2;
-        continue;
-      }
-    }
-    if (q[0] != '/' || p != s || isabs) {
-      *p++ = *q;
-    }
-  }
-  if (p == s) {
-    *p++ = isabs ? '/' : '.';
-  }
-  if (p == s + 1 && s[0] == '.') {
-    *p++ = '/';
-  }
-  while (p - s > 1 && p[-1] == '/') {
-    --p;
-  }
-  *p = '\0';
-  return p - s;
+  return p - d;
 }