Improve environment var handling (#22796)

* refactor env var handling

- use shared helper
- use left-to-right parser

* add tests

* formatting

frigate/config/env.py

@@ -1,4 +1,5 @@
 import os
+import re
 from pathlib import Path
 from typing import Annotated
 
@@ -15,8 +16,77 @@ if os.path.isdir(secrets_dir) and os.access(secrets_dir, os.R_OK):
             )
 
 
+# Matches a FRIGATE_* identifier following an opening brace.
+_FRIGATE_IDENT_RE = re.compile(r"FRIGATE_[A-Za-z0-9_]+")
+
+
+def substitute_frigate_vars(value: str) -> str:
+    """Substitute `{FRIGATE_*}` placeholders in *value*.
+
+    Reproduces the subset of `str.format()` brace semantics that Frigate's
+    config has historically supported, while leaving unrelated brace content
+    (e.g. ffmpeg `%{localtime\\:...}` expressions) untouched:
+
+    * `{{` and `}}` collapse to literal `{` / `}` (the documented escape).
+    * `{FRIGATE_NAME}` is replaced from `FRIGATE_ENV_VARS`; an unknown name
+      raises `KeyError` to preserve the existing "Invalid substitution"
+      error path.
+    * A `{` that begins `{FRIGATE_` but is not a well-formed
+      `{FRIGATE_NAME}` placeholder raises `ValueError` (malformed
+      placeholder). Callers that catch `KeyError` to allow unknown-var
+      passthrough will still surface malformed syntax as an error.
+    * Any other `{` or `}` is treated as a literal and passed through.
+    """
+    out: list[str] = []
+    i = 0
+    n = len(value)
+    while i < n:
+        ch = value[i]
+        if ch == "{":
+            # Escaped literal `{{`.
+            if i + 1 < n and value[i + 1] == "{":
+                out.append("{")
+                i += 2
+                continue
+            # Possible `{FRIGATE_*}` placeholder.
+            if value.startswith("{FRIGATE_", i):
+                ident_match = _FRIGATE_IDENT_RE.match(value, i + 1)
+                if (
+                    ident_match is not None
+                    and ident_match.end() < n
+                    and value[ident_match.end()] == "}"
+                ):
+                    key = ident_match.group(0)
+                    if key not in FRIGATE_ENV_VARS:
+                        raise KeyError(key)
+                    out.append(FRIGATE_ENV_VARS[key])
+                    i = ident_match.end() + 1
+                    continue
+                # Looks like a FRIGATE placeholder but is malformed
+                # (no closing brace, illegal char, format spec, etc.).
+                raise ValueError(
+                    f"Malformed FRIGATE_ placeholder near {value[i : i + 32]!r}"
+                )
+            # Plain `{` — pass through (e.g. `%{localtime\:...}`).
+            out.append("{")
+            i += 1
+            continue
+        if ch == "}":
+            # Escaped literal `}}`.
+            if i + 1 < n and value[i + 1] == "}":
+                out.append("}")
+                i += 2
+                continue
+            out.append("}")
+            i += 1
+            continue
+        out.append(ch)
+        i += 1
+    return "".join(out)
+
+
 def validate_env_string(v: str) -> str:
-    return v.format(**FRIGATE_ENV_VARS)
+    return substitute_frigate_vars(v)
 
 
 EnvString = Annotated[str, AfterValidator(validate_env_string)]