resolves #188: git-ship with no shippable changes

features/git-ship/current_branch/errors.feature

@@ -1,12 +1,5 @@
 Feature: Git Ship: errors while shipping the current branch
 
-  Scenario: does not ship a feature branch not ahead of main
-    Given I am on a feature branch
-    When I run `git ship -m 'feature done'` while allowing errors
-    Then I get the error "The branch 'feature' has no commits to merge into 'main'."
-    And I end up on the "feature" branch
-
-
   Scenario: does not ship the main branch
     Given I am on the main branch
     When I run `git ship -m 'feature done'` while allowing errors

features/git-ship/current_branch/no_diff.feature

@@ -0,0 +1,15 @@
+Feature: Git Ship: errors when the branch diff is empty
+
+  Background:
+    Given I have a feature branch named "feature"
+    And the following commit exists in my repository
+      | branch  | location | file name   | file content   |
+      | main    | remote   | common_file | common content |
+      | feature | local    | common_file | common content |
+    And I am on the "feature" branch
+    When I run `git ship -m 'feature done'` while allowing errors
+
+
+  Scenario: result
+    Then I get the error "The branch 'feature' has no shippable changes"
+    And I am still on the "feature" branch

features/git-ship/supplied_branch/errors_with_open_changes.feature

@@ -10,16 +10,6 @@ Feature: Git Ship: errors while shipping the supplied branch with open changes
     And I still have an uncommitted file with name: "uncommitted" and content: "stuff"
 
 
-  Scenario: does not ship a feature branch that is not ahead of main
-    Given I have feature branches named "feature" and "other_feature"
-    And I am on the "other_feature" branch
-    And I have an uncommitted file with name: "uncommitted" and content: "stuff"
-    When I run `git ship feature -m 'feature done'` while allowing errors
-    Then I get the error "The branch 'feature' has no commits to merge into 'main'."
-    And I end up on the "other_feature" branch
-    And I still have an uncommitted file with name: "uncommitted" and content: "stuff"
-
-
   Scenario: does not ship the main branch
     Given I am on the "feature" branch
     And I have an uncommitted file with name: "uncommitted" and content: "stuff"

features/git-ship/supplied_branch/errors_without_open_changes.feature

@@ -8,14 +8,6 @@ Feature: Git Ship: errors while shipping the supplied branch without open change
     And I end up on the "feature" branch
 
 
-  Scenario: does not ship a feature branch that is not ahead of main
-    Given I have feature branches named "feature" and "other_feature"
-    And I am on the "other_feature" branch
-    When I run `git ship feature -m 'feature done'` while allowing errors
-    Then I get the error "The branch 'feature' has no commits to merge into 'main'."
-    And I end up on the "other_feature" branch
-
-
   Scenario: does not ship the main branch
     Given I am on the "feature" branch
     When I run `git ship main -m 'feature done'` while allowing errors

features/git-ship/supplied_branch/no_diff_with_open_changes.feature

@@ -0,0 +1,17 @@
+Feature: Git Ship: errors when the branch diff is empty with open changes
+
+  Background:
+    Given I have feature branches named "feature" and "other_feature"
+    And the following commit exists in my repository
+      | branch  | location | file name   | file content   |
+      | main    | remote   | common_file | common content |
+      | feature | local    | common_file | common content |
+    And I am on the "other_feature" branch
+    And I have an uncommitted file with name: "uncommitted" and content: "stuff"
+    When I run `git ship feature` while allowing errors
+
+
+  Scenario: result
+    Then I get the error "The branch 'feature' has no shippable changes"
+    And I am still on the "other_feature" branch
+    And I still have an uncommitted file with name: "uncommitted" and content: "stuff"

features/git-ship/supplied_branch/no_diff_without_open_changes.feature

@@ -0,0 +1,15 @@
+Feature: Git Ship: errors when the branch diff is empty without open changes
+
+  Background:
+    Given I have feature branches named "feature" and "other_feature"
+    And the following commit exists in my repository
+      | branch  | location | file name   | file content   |
+      | main    | remote   | common_file | common content |
+      | feature | local    | common_file | common content |
+    And I am on the "other_feature" branch
+    When I run `git ship feature` while allowing errors
+
+
+  Scenario: result
+    Then I get the error "The branch 'feature' has no shippable changes"
+    And I am still on the "other_feature" branch

features/support/run_helpers.rb

@@ -42,7 +42,7 @@ def run command, allow_failures: false, debug: false, input: nil
 def run_shell_command command, input
   result = OpenStruct.new(command: command, location: Dir.pwd.split(/[_\/]/).last)
 
-  status = Open4.popen4("#{command} 2>&1") do |_pid, stdin, stdout, stderr|
+  status = Open4.popen4("#{command} 2>&1") do |_pid, stdin, stdout, _stderr|
     stdin.puts input if input
     stdin.close
     result.out = stdout.read

git-ship

@@ -5,19 +5,6 @@ source "$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )/helpers/helpers.sh"
 export abort_script_filename=$(abort_script_filename_for_command ship)
 
 
-
-# Exit if the current branch is not ahead of main
-function ensure_ahead_of_main_branch {
-  local current_branch_name=$(get_current_branch_name)
-  if [ "$(is_ahead_of_main "$current_branch_name")" == false ]; then
-    return_to_initial_branch
-
-    echo_error_header
-    echo_error "The branch '$current_branch_name' has no commits to merge into '$main_branch_name'."
-    exit_with_error
-  fi
-}
-
 # Called when squash_merge fails due to empty commit message
 function error_empty_commit {
   discard_open_changes
@@ -95,7 +82,7 @@ function perform_ship {
   checkout_branch "$target_branch_name"
   pull_branch
   merge_branch "$main_branch_name"
-  ensure_ahead_of_main_branch
+  ensure_has_shippable_changes
   checkout_main_branch
   squash_merge "$target_branch_name" "$message"
   push_branch

helpers/git_helpers.sh

@@ -146,6 +146,19 @@ function ensure_has_branch {
 }
 
 
+# Exit if the current branch does not have shippable changes
+function ensure_has_shippable_changes {
+  local current_branch_name=$(get_current_branch_name)
+  if [ "$(has_shippable_changes "$current_branch_name")" == false ]; then
+    return_to_initial_branch
+
+    echo_error_header
+    echo_error "The branch '$current_branch_name' has no shippable changes."
+    exit_with_error
+  fi
+}
+
+
 # Exits the application with an error message if the supplied branch is
 # not a feature branch
 function ensure_is_feature_branch {
@@ -262,21 +275,21 @@ function has_open_changes {
 }
 
 
-# Determines whether the given branch has a remote tracking branch.
-function has_tracking_branch {
+# Determines whether the given branch has shippable changes
+function has_shippable_changes {
   local branch_name=$1
-  if [ "$(git branch -r | tr -d ' ' | grep -c "^origin\/$branch_name\$")" == 0 ]; then
+  if [ "$(git diff --quiet "$main_branch_name..$branch_name" ; echo $?)" == 0 ]; then
     echo false
   else
     echo true
   fi
 }
 
 
-# Determines whether the given branch is ahead of main
-function is_ahead_of_main {
+# Determines whether the given branch has a remote tracking branch.
+function has_tracking_branch {
   local branch_name=$1
-  if [ "$(git log --oneline "$main_branch_name..$branch_name" | wc -l | tr -d ' ')" == 0 ]; then
+  if [ "$(git branch -r | tr -d ' ' | grep -c "^origin\/$branch_name\$")" == 0 ]; then
     echo false
   else
     echo true