Add compile-time syntax validation for concurrency group expressions by Copilot · Pull Request #15082 · github/gh-aw

Copilot · 2026-02-12T03:39:32Z

Custom concurrency group expressions can contain GitHub Actions expressions like ${{ github.ref }}. Previously, syntax errors (unbalanced braces, unclosed quotes, malformed operators) were only caught at runtime.

Changes

New validation module (concurrency_validation.go)
- Validates ${{ }} brace balance and nesting
- Checks parentheses and quote pairing (single, double, backtick)
- Detects malformed logical operators (&&, ||, !)
- Leverages existing expression parser for complex boolean logic
Compiler integration (compiler.go)
- Validates both workflow-level and engine-level concurrency during validateWorkflowData()
- Handles string format (concurrency: "group") and object format (concurrency: {group: "group"})

Example

Invalid expression:

concurrency: workflow-${{ github.ref

Compile error:

✗ workflow-level concurrency validation failed: unclosed expression braces
  found opening '${{' at position 9 without matching closing '}}'
  Suggestion: Add the missing closing braces.

Valid expressions pass through:

concurrency: workflow-${{ github.ref }}
concurrency: gh-aw-${{ github.workflow }}-${{ github.event.pr.number || github.ref }}

Original prompt

Add compiler support to validate the syntax of custom group expressions as thoroughly as possible and detect obvious syntactic errors at compile time.

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Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

pelikhan · 2026-02-12T04:08:57Z

At @copilot, add more tests.

Copilot

Pull request overview

Adds compile-time validation for GitHub Actions-style ${{ }} expressions used in custom concurrency group strings, surfacing common syntax issues during compilation instead of at runtime.

Changes:

Introduces a new validation module for concurrency group expressions (braces, parentheses/quotes, and parsing for boolean logic).
Integrates validation into the compiler for both workflow-level and engine-level concurrency configuration.
Adds unit + integration tests (and benchmarks) covering common valid/invalid patterns and error message quality.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
pkg/workflow/concurrency_validation.go	Implements concurrency group expression extraction + syntax validation logic.
pkg/workflow/compiler.go	Calls the new validator during compile-time workflow data validation for workflow/engine concurrency.
pkg/workflow/concurrency_validation_test.go	Adds unit tests and benchmarks for the new validation helpers and extraction logic.
pkg/workflow/concurrency_validation_integration_test.go	Adds integration tests asserting compile-time failures/successes for concurrency syntax.

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Copilot · 2026-02-12T04:17:21Z

pkg/workflow/concurrency_validation.go

+	// Pattern to extract content between ${{ and }}
+	expressionPattern := regexp.MustCompile(`\$\{\{([^}]*)\}\}`)


The extraction regex \\$\\{\\{([^}]*)\\}\\} fails to match valid expressions that contain a } character inside the expression (e.g., string literals or function calls producing/containing braces). In that case, matches becomes empty and the validator silently skips validating expression content (parentheses/quotes/parser), allowing syntax errors to pass. Consider replacing this regex-based extraction with a small scanner that finds ${{ and the corresponding }} using index searching (and/or reusing the brace-balance traversal) so expressions are always extracted and validated correctly.

Suggested change

// Pattern to extract content between ${{ and }}

expressionPattern := regexp.MustCompile(`\$\{\{([^}]*)\}\}`)

// Pattern to extract content between ${{ and }}.

// Use a non-greedy wildcard so that '}' characters inside the expression are allowed.

expressionPattern := regexp.MustCompile(`\$\{\{(.*?)\}\}`)

Copilot · 2026-02-12T04:17:22Z

pkg/workflow/concurrency_validation.go

+	// Check for unbalanced parentheses
+	parenCount := 0
+	for i, ch := range expr {
+		switch ch {
+		case '(':
+			parenCount++
+		case ')':
+			parenCount--
+			if parenCount < 0 {
+				return NewValidationError(
+					"concurrency",
+					"unbalanced parentheses in expression",
+					fmt.Sprintf("found closing ')' without matching opening '(' at position %d in expression: %s", i, expr),
+					"Ensure all parentheses are properly balanced in your concurrency group expression.",
+				)
+			}
+		}
+	}


Parentheses are counted without considering whether they occur inside quoted strings/backticks. This can produce false positives for expressions like github.ref == '(' or format('(%)', ...), where parentheses inside quotes should not affect balance. A tangible fix is to perform the parentheses scan while tracking quote state (single/double/backtick + escapes), skipping ( and ) when inside any quote context.

Copilot · 2026-02-12T04:17:22Z

pkg/workflow/concurrency_validation.go

+	for i, ch := range expr {
+		if escaped {
+			escaped = false
+			continue
+		}
+
+		if ch == '\\' {
+			escaped = true
+			continue
+		}
+
+		switch ch {
+		case '\'':
+			if !inDoubleQuote && !inBacktick {
+				inSingleQuote = !inSingleQuote
+			}
+		case '"':
+			if !inSingleQuote && !inBacktick {
+				inDoubleQuote = !inDoubleQuote
+			}
+		case '`':
+			if !inSingleQuote && !inDoubleQuote {
+				inBacktick = !inBacktick
+			}
+		}
+
+		// Check if we reached end of string with unclosed quote
+		if i == len(expr)-1 {
+			if inSingleQuote {
+				return NewValidationError(
+					"concurrency",
+					"unclosed single quote",
+					fmt.Sprintf("found unclosed single quote in expression: %s", expr),
+					"Add the missing closing single quote (') to your expression.",
+				)
+			}
+			if inDoubleQuote {
+				return NewValidationError(
+					"concurrency",
+					"unclosed double quote",
+					fmt.Sprintf("found unclosed double quote in expression: %s", expr),
+					"Add the missing closing double quote (\") to your expression.",
+				)
+			}
+			if inBacktick {
+				return NewValidationError(
+					"concurrency",
+					"unclosed backtick",
+					fmt.Sprintf("found unclosed backtick in expression: %s", expr),
+					"Add the missing closing backtick (`) to your expression.",
+				)
+			}
+		}
+	}
+
+	return nil


Unclosed quotes can be missed when the final character is a backslash. In that case the loop hits ch == '\\\\', sets escaped = true, continues, and never runs the i == len(expr)-1 unclosed-quote checks. A concrete fix is to move 'unclosed quote' checks after the loop (based on inSingleQuote/inDoubleQuote/inBacktick), rather than relying on the last-iteration branch inside the loop.

Copilot · 2026-02-12T04:17:22Z

pkg/workflow/concurrency_validation.go

+	// Try to parse complex expressions with logical operators
+	if containsLogicalOperators(expr) {
+		if _, err := ParseExpression(expr); err != nil {
+			return NewValidationError(
+				"concurrency",
+				"invalid expression syntax",
+				fmt.Sprintf("failed to parse expression in concurrency group: %s", err.Error()),
+				fmt.Sprintf("Fix the syntax error in your concurrency group expression. Full expression: %s", fullGroup),
+			)
+		}
+	}
+


Only parsing expressions when containsLogicalOperators(expr) is true leaves a gap where syntactically invalid expressions without &&, ||, or ! are accepted (e.g., github.ref == or other malformed comparisons/calls). If ParseExpression is intended to be the source of truth for syntax, it should be invoked for all extracted ${{ ... }} expressions after basic brace/quote checks (or at least for a broader set of operator tokens than just logical ones).

Suggested change

// Try to parse complex expressions with logical operators

if containsLogicalOperators(expr) {

if _, err := ParseExpression(expr); err != nil {

return NewValidationError(

"concurrency",

"invalid expression syntax",

fmt.Sprintf("failed to parse expression in concurrency group: %s", err.Error()),

fmt.Sprintf("Fix the syntax error in your concurrency group expression. Full expression: %s", fullGroup),

)

}

}

// Parse all non-empty expressions to validate syntax

trimmedExpr := strings.TrimSpace(expr)

if trimmedExpr == "" {

return nil

}

if _, err := ParseExpression(trimmedExpr); err != nil {

return NewValidationError(

"concurrency",

"invalid expression syntax",

fmt.Sprintf("failed to parse expression in concurrency group: %s", err.Error()),

fmt.Sprintf("Fix the syntax error in your concurrency group expression. Full expression: %s", fullGroup),

)

}

Initial plan

8483a43

Copilot AI assigned Copilot and pelikhan Feb 12, 2026

Copilot started work on behalf of pelikhan February 12, 2026 03:39 View session

Copilot AI and others added 4 commits February 12, 2026 03:51

Add concurrency group expression validation with tests

21816cf

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

Fix lint errors and improve concurrency validation

e2bc4de

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

Address code review feedback - improve documentation

ab7ffe5

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

Final documentation improvements based on code review

ceefc87

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

Copilot AI changed the title ~~[WIP] Add compiler support for validating custom group expressions~~ Add compile-time syntax validation for concurrency group expressions Feb 12, 2026

Copilot finished work on behalf of pelikhan February 12, 2026 04:05

Copilot AI requested a review from pelikhan February 12, 2026 04:05

Copilot started work on behalf of pelikhan February 12, 2026 04:09 View session

pelikhan marked this pull request as ready for review February 12, 2026 04:13

Copilot AI review requested due to automatic review settings February 12, 2026 04:13

pelikhan merged commit 77f63b8 into main Feb 12, 2026
2 of 3 checks passed

pelikhan deleted the copilot/add-compiler-support-syntax-validation branch February 12, 2026 04:13

Copilot started reviewing on behalf of pelikhan February 12, 2026 04:13 View session

Copilot stopped work on behalf of pelikhan due to an error February 12, 2026 04:13
The session was cancelled by the user.

Copilot AI reviewed Feb 12, 2026

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Add compile-time syntax validation for concurrency group expressions#15082

Add compile-time syntax validation for concurrency group expressions#15082
pelikhan merged 5 commits intomainfrom
copilot/add-compiler-support-syntax-validation

Copilot AI commented Feb 12, 2026 •

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		// Pattern to extract content between ${{ and }}
		expressionPattern := regexp.MustCompile(`\$\{\{([^}]*)\}\}`)

-	// Pattern to extract content between ${{ and }}
-	expressionPattern := regexp.MustCompile(`\$\{\{([^}]*)\}\}`)
+	// Pattern to extract content between ${{ and }}.
+	// Use a non-greedy wildcard so that '}' characters inside the expression are allowed.
+	expressionPattern := regexp.MustCompile(`\$\{\{(.*?)\}\}`)

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Copilot AI commented Feb 12, 2026 •

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