Merge branch 'zero-scenario-fail' into 'main'

Ensure codegen barfs on zero scenario documents

Closes #236

See merge request subplot/subplot!219

4 files changed, 45 insertions, 4 deletions

diff --git a/reference.md b/reference.md
index 1488ce0..62f827b 100644
--- a/reference.md
+++ b/reference.md
@@ -11,7 +11,7 @@ documents.
 
 ~~~scenario
 given an installed subplot
-given a clone of https://gitlab.com/subplot/subplot.git in src at 8fe4a393ccbda6820b57ec873f37f08ce17d7c3d
+given a clone of https://gitlab.com/subplot/subplot.git in src at 409f32c5017d3f880e003138b0548cf74f965ce0
 when I docgen subplot.md to test.pdf, in src
 then file src/test.pdf exists
 ~~~
@@ -20,7 +20,7 @@ then file src/test.pdf exists
 
 ~~~scenario
 given an installed subplot
-given a clone of https://gitlab.com/subplot/subplot.git in src at 8fe4a393ccbda6820b57ec873f37f08ce17d7c3d
+given a clone of https://gitlab.com/subplot/subplot.git in src at 409f32c5017d3f880e003138b0548cf74f965ce0
 when I docgen subplot.md to test.html, in src
 when I run, in src, subplot docgen subplot.md -o subplot.html
 then file src/test.html exists
@@ -31,7 +31,7 @@ then file src/test.html exists
 ~~~scenario
 given an installed subplot
 given file run_test.sh
-given a clone of https://gitlab.com/subplot/subplot.git in src at 8fe4a393ccbda6820b57ec873f37f08ce17d7c3d
+given a clone of https://gitlab.com/subplot/subplot.git in src at 409f32c5017d3f880e003138b0548cf74f965ce0
 when I run, in src, subplot codegen subplot.md -o test-inner.py
 when I run bash run_test.sh
 then command is successful
diff --git a/src/codegen.rs b/src/codegen.rs
index 89ea591..b67d94a 100644
--- a/src/codegen.rs
+++ b/src/codegen.rs
@@ -39,7 +39,8 @@ pub fn generate_test_program(
 
 fn context(doc: &mut Document, template: &str) -> Result<Context, SubplotError> {
     let mut context = Context::new();
-    context.insert("scenarios", &doc.matched_scenarios(template)?);
+    let scenarios = doc.matched_scenarios(template)?;
+    context.insert("scenarios", &scenarios);
     context.insert("files", doc.files());
 
     let funcs_filenames = doc.meta().functions_filenames();
@@ -51,6 +52,11 @@ fn context(doc: &mut Document, template: &str) -> Result<Context, SubplotError>
     }
     context.insert("functions", &funcs);
 
+    // Any of the above could fail for more serious reasons, but if we get this far
+    // and our context would have no scenarios in it, then we complain.
+    if scenarios.is_empty() {
+        return Err(SubplotError::NoScenariosMatched(template.to_string()));
+    }
     Ok(context)
 }
 
diff --git a/src/error.rs b/src/error.rs
index e69bffe..c041eaf 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -233,6 +233,10 @@ pub enum SubplotError {
     #[error("failure rendering pikchr diagram: {0}")]
     PikchrRenderError(String),
 
+    /// When attempting to codegen, no scenarios matched the desired template language
+    #[error("no scenarios were found matching the `{0}` template")]
+    NoScenariosMatched(String),
+
     /// I/O error
     ///
     /// Subplot did some I/O, and it failed. This is a generic wrapper
diff --git a/subplot.md b/subplot.md
index a718490..2d7d760 100644
--- a/subplot.md
+++ b/subplot.md
@@ -1071,6 +1071,30 @@ and step "then bar was done" was run
 and command is successful
 ~~~
 
+## No scenarios means codegen fails
+
+If you attempt to `subplot codegen` on a document which contains no scenarios, the
+tool will fail to execute with a reasonable error message.
+
+~~~scenario
+given file noscenarios.md
+and an installed subplot
+when I try to run subplot codegen noscenarios.md -o test.py
+then command fails
+and stderr contains "no scenarios were found"
+~~~
+
+~~~{#noscenarios.md .file .markdown .numberLines}
+---
+title: No scenarios in here
+template: python
+...
+
+# This is a title
+
+But there are no scenarios in this file, and thus nothing can be generated in a test suite.
+
+~~~
 
 ## Keywords
 
@@ -2099,6 +2123,13 @@ template: python
 
 This is a very simple Markdown file that uses every kind of inline
 markup in the title and chapter heading.
+
+To satisfy codegen, we *MUST* have a scenario here
+~~~~scenario
+when I do bar
+then bar was done
+~~~~
+
 ~~~