feat: add __RUN_ID__ substitution variable

DuckDB derives an attached database name from the file basename, ignoring
the directory. A test re-run against a shared server (e.g. once per engine)
therefore reuses the basename and fails "database already exists" on a plain
ATTACH without AS. __RUN_ID__ expands to the per-run temp-dir random suffix
(stable within a run, unique across runs), so embedding it in ATTACH file
names makes the derived name unique per run.

Author: MBkkt

sqllogictest/src/runner.rs

@@ -670,8 +670,8 @@ pub(crate) struct RunnerLocals {
 }
 
 impl RunnerLocals {
-    pub fn test_dir(&self) -> String {
-        let test_dir = self.test_dir.get_or_init(|| {
+    fn test_dir_handle(&self) -> &TempDir {
+        self.test_dir.get_or_init(|| {
             let dir = TempDir::new().expect("failed to create testdir");
             // When the runner and the database server run in separate containers,
             // server-side ops (COPY TO/FROM, ATTACH) write into __TEST_DIR__ as a
@@ -684,8 +684,30 @@ impl RunnerLocals {
                     .expect("failed to relax testdir permissions");
             }
             dir
-        });
-        test_dir.path().to_string_lossy().into_owned()
+        })
+    }
+
+    pub fn test_dir(&self) -> String {
+        self.test_dir_handle().path().to_string_lossy().into_owned()
+    }
+
+    /// A short alphanumeric token unique to this runner instance (derived from the
+    /// random temp-dir name), stable for the whole run. DuckDB derives an attached
+    /// database's name from the file basename only, ignoring the directory, so the
+    /// same test re-run against a shared server (e.g. once per engine) would reuse
+    /// the basename and collide ("database already exists"). Embedding `__RUN_ID__`
+    /// in the file name makes the derived name unique per run.
+    pub fn run_id(&self) -> String {
+        self.test_dir_handle()
+            .path()
+            .file_name()
+            .map(|name| {
+                name.to_string_lossy()
+                    .chars()
+                    .filter(|c| c.is_ascii_alphanumeric())
+                    .collect()
+            })
+            .unwrap_or_default()
     }
 
     fn set_var(&mut self, key: String, value: String) {

sqllogictest/src/substitution.rs

@@ -4,6 +4,7 @@ use crate::RunnerLocals;
 
 pub mod well_known {
     pub const TEST_DIR: &str = "__TEST_DIR__";
+    pub const RUN_ID: &str = "__RUN_ID__";
     pub const NOW: &str = "__NOW__";
     pub const DATABASE: &str = "__DATABASE__";
 }
@@ -80,6 +81,10 @@ impl Substitution<'_> {
                 &format!("${}", well_known::TEST_DIR),
                 &self.runner_locals.test_dir(),
             )
+            .replace(
+                &format!("${}", well_known::RUN_ID),
+                &self.runner_locals.run_id(),
+            )
             .replace(&format!("${}", well_known::NOW), &now_string());
         for (key, value) in self.runner_locals.vars() {
             res = res.replace(&format!("${}", key), value);
@@ -94,6 +99,7 @@ impl<'a> subst::VariableMap<'a> for Substitution<'a> {
     fn get(&'a self, key: &str) -> Option<Self::Value> {
         match key {
             well_known::TEST_DIR => self.runner_locals.test_dir().into(),
+            well_known::RUN_ID => self.runner_locals.run_id().into(),
             well_known::NOW => now_string().into(),
             key => self
                 .runner_locals