Keep the empty host for a state_dir starting with //

With OmitHost set, a state_dir starting with // gave a db_url starting
file://, and SQLite read the first directory name as a host, so pixa
failed to start. OmitHost is now left off for such a directory: the URL
starts file://// and SQLite reads an empty host followed by the path.
Every other state_dir gives the same URL as before.

Model: opus-5-5

README.md

@@ -726,12 +726,13 @@ Key settings in more detail:
 - `db_url` — the SQLite database to open; omitted, it is
   `file:<state_dir>/state.sqlite3?_pragma=journal_mode(WAL)`, which keeps the
   database in WAL mode. `state_dir` is percent-encoded in it, so a `%`, `?` or
-  `#` in the name is written as `%25`, `%3F` or `%23`. pixa opens one connection
+  `#` in the name is written as `%25`, `%3F` or `%23`. For a `state_dir`
-  to it, so its own reads and writes run one at a time. pixa adds
+  starting with `//` it starts `file:////`, so SQLite does not read the first
-  `_pragma=busy_timeout(5000)` to any `db_url`, so a write that finds another
+  directory name as a host. pixa opens one connection to it, so its own reads
-  program writing to the file waits up to five seconds for it instead of
+  and writes run one at a time. pixa adds `_pragma=busy_timeout(5000)` to any
-  failing. WAL mode comes only from the URL: keep `_pragma=journal_mode(WAL)` in
+  `db_url`, so a write that finds another program writing to the file waits up
-  one you set
+  to five seconds for it instead of failing. WAL mode comes only from the URL:
+  keep `_pragma=journal_mode(WAL)` in one you set
 - `cache_max_bytes` — disk cache size limit in bytes; `0` disables the disk
   cache entirely; omitted defaults to 75% of the sum of the free space on the
   filesystem containing `<state_dir>/cache/` and the bytes of source and

TODO.md

@@ -67,7 +67,9 @@ P2: security: per-origin rate limiting
   it put the name into a `file:` URL as it was, so SQLite read `%41` as `A`, a
   `?` or `#` ended the path early, and the journal mode parameter was lost. The
   derived `db_url` is now built with Go's `net/url`, which writes those
-  characters as `%25`, `%3F` and `%23`.
+  characters as `%25`, `%3F` and `%23`. For a `state_dir` starting with `//` it
+  starts `file:////`, so SQLite no longer reads the first directory name as a
+  host.
 - 2026-10-15 a thin source requested with `fit=cover` no longer makes libvips
   build a huge image (closes #282): libvips' thumbnail scaled the whole source
   to cover the requested size and copied that into memory before it cut out the

internal/config/config.go

@@ -359,11 +359,13 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 // the directory name, which SQLite decodes back, so the URL names exactly
 // this file. OmitHost writes file:/var/lib/pixa/... instead of
 // file:///var/lib/pixa/..., and keeps a relative stateDir from being read
-// as a host name.
+// as a host name. A stateDir starting with // keeps the empty host, so the
+// URL starts file://// and SQLite does not read the first directory name
+// as a host.
 func defaultDBURL(stateDir string) string {
 	dbURL := url.URL{
 		Scheme:   "file",
-		OmitHost: true,
+		OmitHost: !strings.HasPrefix(stateDir, "//"),
 		Path:     stateDir + "/state.sqlite3",
 		RawQuery: "_pragma=journal_mode(WAL)",
 	}