submit: Use collision-resolved upstream bases (#1430)

When `stack submit` encounters remote branches with the same names as
local stack members, it allocates collision-free upstream names.
Upper pull requests still used the occupied local names as their bases,
so the first submission produced an incorrect pull request chain.

Carry each selected upstream name through the batch and prefer it over
the preloaded graph snapshot when submitting the next branch.
This preserves graph reuse while creating the correct base chain on the
first submission.

Fixes #1394

.changes/unreleased/Fixed-20260830-123454.yaml

@@ -0,0 +1,3 @@
+kind: Fixed
+body: 'stack submit: Pull requests are now created with the correct base branches when existing remote branches have the same names as the local stack.'
+time: 2026-08-30T12:34:54.354962-07:00

internal/handler/submit/handler.go

@@ -236,19 +236,29 @@ func (h *Handler) SubmitBatch(ctx context.Context, req *BatchRequest) error {
 		return err
 	}
 
+	resolvedUpstreams := make(map[string]string)
 	var branchesToComment []string
 	for _, branch := range req.Branches {
+		branchInfo, ok := graph.Lookup(branch)
+		if !ok {
+			return fmt.Errorf("submit branch %s: lookup branch: %w", branch, state.ErrNotExist)
+		}
+
 		// Shallow copy the options because submitBranch may modify them.
 		opts := *opts
 		status, err := h.submitBranch(
 			ctx,
 			graph,
 			branch,
-			&submitOptions{Options: &opts},
+			&submitOptions{
+				Options:      &opts,
+				upstreamBase: resolvedUpstreams[branchInfo.Base],
+			},
 		)
 		if err != nil {
 			return fmt.Errorf("submit branch %s: %w", branch, err)
 		}
+		resolvedUpstreams[branch] = status.upstreamBranch
 		if status.Submitted {
 			branchesToComment = append(branchesToComment, branch)
 		}
@@ -347,12 +357,15 @@ type submitStatus struct {
 	// If yes, comments will be added or updated
 	// based on the NavComment option.
 	Submitted bool
+
+	upstreamBranch string
 }
 
 type submitOptions struct {
 	*Options
 
-	Title, Body string
+	Title, Body  string
+	upstreamBase string
 }
 
 func (h *Handler) submitBranch(
@@ -405,6 +418,9 @@ func (h *Handler) submitBranch(
 	if err != nil {
 		return status, fmt.Errorf("resolve upstream branch: %w", err)
 	}
+	defer func() {
+		status.upstreamBranch = upstreamBranch
+	}()
 
 	// Similarly, if the branch's base has a different name upstream,
 	// use that name instead.
@@ -414,7 +430,7 @@ func (h *Handler) submitBranch(
 		if !ok {
 			return status, fmt.Errorf("lookup base branch: %w", state.ErrNotExist)
 		}
-		upstreamBase = cmp.Or(baseBranch.UpstreamBranch, branch.Base)
+		upstreamBase = cmp.Or(opts.upstreamBase, baseBranch.UpstreamBranch, branch.Base)
 	}
 
 	var existingChange *forge.FindChangeItem

testdata/script/issue1394_stack_submit_resolved_upstream_bases.txt

@@ -0,0 +1,118 @@
+# 'stack submit' uses collision-resolved upstream names as PR bases.
+#
+# https://github.com/abhinav/git-spice/issues/1394
+
+as 'Test <test@example.com>'
+at '2026-08-30T19:28:02Z'
+
+# Setup.
+cd repo
+git init
+git commit --allow-empty -m 'Initial commit'
+
+# Set up a fake GitHub remote with the desired branch names already taken.
+shamhub-setup
+shamhub register alice
+shamhub new origin alice/example.git
+git push origin main main:float-1 main:float-2 main:float-3
+gs repo init
+
+env SHAMHUB_USERNAME=alice
+gs auth login
+
+# Create the local float-1 -> float-2 -> float-3 stack.
+git add float-1.txt
+gs branch create float-1 -m 'Add float 1'
+git add float-2.txt
+gs branch create float-2 -m 'Add float 2'
+git add float-3.txt
+gs branch create float-3 -m 'Add float 3'
+
+# Each local branch needs a collision-free upstream name.
+# Upper PRs must target those resolved names during the same submission.
+gs stack submit --fill
+stderr 'float-1: Using upstream name ''float-1-2'' instead'
+stderr 'float-2: Using upstream name ''float-2-2'' instead'
+stderr 'float-3: Using upstream name ''float-3-2'' instead'
+
+shamhub dump changes
+cmpenvJSON stdout $WORK/golden/changes.json
+
+-- repo/float-1.txt --
+Float 1
+-- repo/float-2.txt --
+Float 2
+-- repo/float-3.txt --
+Float 3
+-- golden/changes.json --
+[
+  {
+    "number": 1,
+    "html_url": "$SHAMHUB_URL/alice/example/change/1",
+    "state": "open",
+    "title": "Add float 1",
+    "body": "",
+    "base": {
+      "repository": {
+        "owner": "alice",
+        "name": "example"
+      },
+      "ref": "main",
+      "sha": "41127951a5bcb2b7258946d758cbe8549dde8fd2"
+    },
+    "head": {
+      "repository": {
+        "owner": "alice",
+        "name": "example"
+      },
+      "ref": "float-1-2",
+      "sha": "19b5f4c2b2d7362f0d02649e1351083ebc7a3d42"
+    }
+  },
+  {
+    "number": 2,
+    "html_url": "$SHAMHUB_URL/alice/example/change/2",
+    "state": "open",
+    "title": "Add float 2",
+    "body": "",
+    "base": {
+      "repository": {
+        "owner": "alice",
+        "name": "example"
+      },
+      "ref": "float-1-2",
+      "sha": "19b5f4c2b2d7362f0d02649e1351083ebc7a3d42"
+    },
+    "head": {
+      "repository": {
+        "owner": "alice",
+        "name": "example"
+      },
+      "ref": "float-2-2",
+      "sha": "bb5562b20af515520df4bcc17277fe7cbda2221b"
+    }
+  },
+  {
+    "number": 3,
+    "html_url": "$SHAMHUB_URL/alice/example/change/3",
+    "state": "open",
+    "title": "Add float 3",
+    "body": "",
+    "base": {
+      "repository": {
+        "owner": "alice",
+        "name": "example"
+      },
+      "ref": "float-2-2",
+      "sha": "bb5562b20af515520df4bcc17277fe7cbda2221b"
+    },
+    "head": {
+      "repository": {
+        "owner": "alice",
+        "name": "example"
+      },
+      "ref": "float-3-2",
+      "sha": "d1a14cc8a8ecd178c66856d80489bb09a3e79ccf"
+    }
+  }
+]