fix: dumps and imports that quietly did the wrong thing

src/commands/postgres/server.rs

@@ -1,7 +1,7 @@
 use std::thread;
 use std::time::{Duration, Instant};
 
-use anyhow::{Result, anyhow, bail};
+use anyhow::{Result, bail};
 
 use super::shape::Kind;
 use crate::cmd::{Cmd, PgIsReady, PgRestore, Ps, Psql};
@@ -10,6 +10,8 @@ use crate::project::Project;
 
 const READY_TIMEOUT: Duration = Duration::from_secs(60);
 const POLL_INTERVAL: Duration = Duration::from_secs(1);
+// stands in for an id a dry run has no running container to look up
+const PLANNED_CONTAINER: &str = "<db container>";
 
 pub(super) struct Database {
     pub(super) service: String,
@@ -22,13 +24,24 @@ impl Database {
     pub(super) fn resolve(ctx: &Ctx) -> Result<Self> {
         let compose = Project::resolve(ctx)?;
         let service = compose.postgres()?;
-        let (user, name) = compose.postgres_credentials(&service);
+        let (user, name) = compose.postgres_credentials(&service)?;
 
-        let container = Ps::id_of(&service).capture(ctx)?.trim().to_string();
+        let listed = Ps::id_of(&service).capture(ctx)?;
+        let mut ids = listed.lines().map(str::trim).filter(|id| !id.is_empty());
 
-        if container.is_empty() {
-            return Err(anyhow!("service {service} has no running container"));
-        }
+        let container = match (ids.next(), ids.next()) {
+            (Some(id), None) => id.to_string(),
+            // one id per line: a scaled service has several, and the whole
+            // listing would go to docker exec as though it were a single id
+            (Some(_), Some(_)) => bail!(
+                "service {service} has more than one container running; \
+                 scale it to one first"
+            ),
+            // a dry run only prints a plan, and it is worth printing with the
+            // project down, which is when it is most likely to be asked for
+            (None, _) if ctx.dry_run => PLANNED_CONTAINER.to_string(),
+            (None, _) => bail!("service {service} has no running container"),
+        };
 
         Ok(Self {
             service,