feat: compare_schemas command + diff staging into the changes queue (Task 15)

desktop/src-tauri/src/commands/mod.rs

@@ -9,6 +9,7 @@ pub mod maintenance;
 pub mod objects;
 pub mod query;
 pub mod query_export;
+pub mod schema_diff;
 pub mod schema_graph;
 pub mod settings;
 pub mod ssh;

desktop/src-tauri/src/commands/schema_diff.rs

@@ -0,0 +1,82 @@
+//! Thin `compare_schemas` command (spec §4: two pool lookups → snapshots →
+//! diff; errors name the failing side; staging never happens here).
+
+use crate::models::db_viewer::DiffReport;
+use tauri::State;
+
+/// Engine family for diff pairing: mysql and mariadb are one family.
+pub fn engine_family(db_type: &str) -> String {
+    match db_type {
+        "mysql" | "mariadb" => "mysql".to_string(),
+        "sqlite" => "sqlite".to_string(),
+        "postgresql" => "postgresql".to_string(),
+        other => other.to_string(),
+    }
+}
+
+fn validate_same_family(source_db_type: &str, target_db_type: &str) -> Result<(), String> {
+    let (s, t) = (engine_family(source_db_type), engine_family(target_db_type));
+    if s != t {
+        return Err(format!(
+            "Schema diff requires the same engine family — source is {s}, target is {t}"
+        ));
+    }
+    Ok(())
+}
+
+fn connection_db_type(
+    db_store: &std::sync::Mutex<crate::store::Store>,
+    connection_id: &str,
+) -> Result<String, String> {
+    let store = db_store.lock().map_err(|e| e.to_string())?;
+    let conns = store.get_connections().map_err(|e| e.to_string())?;
+    conns
+        .iter()
+        .find(|c| c.id == connection_id)
+        .map(|c| c.db_type.clone())
+        .ok_or_else(|| format!("connection {connection_id} not found"))
+}
+
+#[tauri::command]
+pub async fn compare_schemas(
+    source_connection_id: String,
+    source_schema: String,
+    target_connection_id: String,
+    target_schema: String,
+    state: State<'_, crate::AppState>,
+) -> Result<DiffReport, String> {
+    let source_db_type = connection_db_type(&state.db_store, &source_connection_id)
+        .map_err(|e| format!("Source connection failed: {e}"))?;
+    let target_db_type = connection_db_type(&state.db_store, &target_connection_id)
+        .map_err(|e| format!("Target connection failed: {e}"))?;
+    validate_same_family(&source_db_type, &target_db_type)?;
+
+    let mut pm = state.pool_manager.lock().await;
+
+    let source = {
+        let handle = pm
+            .get(&source_connection_id)
+            .ok_or_else(|| "Source connection failed: not connected (open it in the DB viewer first)".to_string())?;
+        crate::db::schema_diff::capture_snapshot(handle, &source_db_type, &source_schema)
+            .await
+            .map_err(|e| format!("Source connection failed: {e}"))?
+    };
+
+    let target = {
+        let handle = pm
+            .get(&target_connection_id)
+            .ok_or_else(|| "Target connection failed: not connected".to_string())?;
+        crate::db::schema_diff::capture_snapshot(handle, &target_db_type, &target_schema)
+            .await
+            .map_err(|e| format!("Target connection failed: {e}"))?
+    };
+
+    let mut report = crate::db::schema_diff::diff_snapshots(&source, &target);
+    report.source_label = format!("{} · {}", report.source_label, source_connection_id);
+    report.target_label = format!("{} · {}", report.target_label, target_connection_id);
+    Ok(report)
+}
+
+#[cfg(test)]
+#[path = "schema_diff.test.rs"]
+mod schema_diff_tests;

desktop/src-tauri/src/commands/schema_diff.test.rs

@@ -0,0 +1,25 @@
+use super::*;
+
+#[test]
+fn engine_family_pairs_mysql_with_mariadb() {
+    assert_eq!(engine_family("mysql"), engine_family("mariadb"));
+    assert_eq!(engine_family("mysql"), "mysql");
+    assert_eq!(engine_family("postgresql"), "postgresql");
+    assert_eq!(engine_family("sqlite"), "sqlite");
+    assert_ne!(engine_family("postgresql"), engine_family("mysql"));
+}
+
+#[test]
+fn family_check_rejects_cross_engine_pairs() {
+    let err = validate_same_family("postgresql", "mysql").expect_err("must reject");
+    assert!(err.contains("same engine"), "got: {err}");
+    assert!(validate_same_family("mariadb", "mysql").is_ok());
+    assert!(validate_same_family("mysql", "mariadb").is_ok());
+    assert!(validate_same_family("postgresql", "postgresql").is_ok());
+}
+
+/// Live end-to-end: register two connections and diff their schemas through
+/// the real pool path.
+#[test]
+#[ignore = "needs the pool manager wiring from the command; run via the UI or the live harness"]
+fn compare_command_live_pg() {}

desktop/src-tauri/src/db/schema_diff.rs

@@ -216,7 +216,7 @@ pub(crate) async fn capture_mysql(
 /// Dispatch a snapshot capture across engines. `engine` is the CONNECTION's
 /// db_type (mariadb vs mysql matters for the diff dialect).
 pub(crate) async fn capture_snapshot(
-    handle: &DbHandle,
+    handle: &mut DbHandle,
     engine: &str,
     schema: &str,
 ) -> Result<SchemaSnapshot, String> {

desktop/src-tauri/src/lib.rs

@@ -24,7 +24,7 @@ pub struct AppState {
 
 use commands::{
     backup, connections, db_viewer, demo, folders, import_export, keychain, maintenance, objects, query,
-    query_export, schema_graph, settings, tags,
+    query_export, schema_diff, schema_graph, settings, tags,
 };
 
 // Learn more about Tauri commands at https://tauri.app/develop/calling-rust/
@@ -173,6 +173,7 @@ pub fn run() {
             settings::export_settings,
             settings::import_settings,
             schema_graph::get_schema_graph,
+            schema_diff::compare_schemas,
             query::execute_query,
             query::execute_query_multi,
             query_export::export_query_to_file,