os: add RFC 1123-compliant hostname.isValid (#9276)

std.net.isValidHostname is currently too generous. It considers strings
like ".example.com", "exa..mple.com", and "-example.com" to be valid
hostnames, which is incorrect according to RFC 1123 (the currently
accepted standard).

Until the standard library function is improved, we can use this local
implementation that does follow the RFC 1123 standard.

I asked Claude to perform an audit of the code based on its understand
of the RFC. It suggested some additional test cases and considers the
overall implementation to be robust (its words) and standards compliant.

Ref: https://www.rfc-editor.org/rfc/rfc1123

src/os/hostname.zig

@@ -11,6 +11,91 @@ pub const LocalHostnameValidationError = error{
     Unexpected,
 };
 
+/// Validates a hostname according to [RFC 1123](https://www.rfc-editor.org/rfc/rfc1123)
+///
+/// std.net.isValidHostname is (currently) too generous. It considers strings like
+/// ".example.com", "exa..mple.com", and "-example.com" to be valid hostnames, which
+/// is incorrect.
+pub fn isValid(hostname: []const u8) bool {
+    if (hostname.len == 0) return false;
+    if (hostname[0] == '.') return false;
+
+    // Ignore trailing dot (FQDN). It doesn't count toward our length.
+    const end = if (hostname[hostname.len - 1] == '.') end: {
+        if (hostname.len == 1) return false;
+        break :end hostname.len - 1;
+    } else hostname.len;
+
+    if (end > 253) return false;
+
+    // Hostnames are divided into dot-separated "labels", which:
+    //
+    // - Start with a letter or digit
+    // - Can contain letters, digits, or hyphens
+    // - Must end with a letter or digit
+    // - Have a minimum of 1 character and a maximum of 63
+    var label_start: usize = 0;
+    var label_len: usize = 0;
+    for (hostname[0..end], 0..) |c, i| {
+        switch (c) {
+            '.' => {
+                if (label_len == 0 or label_len > 63) return false;
+                if (!std.ascii.isAlphanumeric(hostname[label_start])) return false;
+                if (!std.ascii.isAlphanumeric(hostname[i - 1])) return false;
+
+                label_start = i + 1;
+                label_len = 0;
+            },
+            '-' => {
+                label_len += 1;
+            },
+            else => {
+                if (!std.ascii.isAlphanumeric(c)) return false;
+                label_len += 1;
+            },
+        }
+    }
+
+    // Validate the final label
+    if (label_len == 0 or label_len > 63) return false;
+    if (!std.ascii.isAlphanumeric(hostname[label_start])) return false;
+    if (!std.ascii.isAlphanumeric(hostname[end - 1])) return false;
+
+    return true;
+}
+
+test isValid {
+    const testing = std.testing;
+
+    // Valid hostnames
+    try testing.expect(isValid("example"));
+    try testing.expect(isValid("example.com"));
+    try testing.expect(isValid("www.example.com"));
+    try testing.expect(isValid("sub.domain.example.com"));
+    try testing.expect(isValid("example.com."));
+    try testing.expect(isValid("host-name.example.com."));
+    try testing.expect(isValid("123.example.com."));
+    try testing.expect(isValid("a-b.com"));
+    try testing.expect(isValid("a.b.c.d.e.f.g"));
+    try testing.expect(isValid("127.0.0.1")); // Also a valid hostname
+    try testing.expect(isValid("a" ** 63 ++ ".com")); // Label exactly 63 chars (valid)
+    try testing.expect(isValid("a." ** 126 ++ "a")); // Total length 253 (valid)
+
+    // Invalid hostnames
+    try testing.expect(!isValid(""));
+    try testing.expect(!isValid(".example.com"));
+    try testing.expect(!isValid("example.com.."));
+    try testing.expect(!isValid("host..domain"));
+    try testing.expect(!isValid("-hostname"));
+    try testing.expect(!isValid("hostname-"));
+    try testing.expect(!isValid("a.-.b"));
+    try testing.expect(!isValid("host_name.com"));
+    try testing.expect(!isValid("."));
+    try testing.expect(!isValid(".."));
+    try testing.expect(!isValid("a" ** 64 ++ ".com")); // Label length 64 (too long)
+    try testing.expect(!isValid("a." ** 126 ++ "ab")); // Total length 254 (too long)
+}
+
 /// Checks if a hostname is local to the current machine. This matches
 /// both "localhost" and the current hostname of the machine (as returned
 /// by `gethostname`).