Add error handling for unclosed code blocks in Markdown inputs

modules/build/src/main/scala/scala/build/internal/markdown/MarkdownCodeBlock.scala

@@ -1,6 +1,8 @@
 package scala.build.internal.markdown
 
 import scala.annotation.tailrec
+import scala.build.Position
+import scala.build.errors.{BuildException, MarkdownUnclosedBackticksError}
 import scala.collection.mutable
 import scala.jdk.CollectionConverters.*
 
@@ -47,12 +49,20 @@ object MarkdownCodeBlock {
 
   /** Finds all code snippets in given input
     *
+    * @param subPath
+    *   the project [[os.SubPath]] to the Markdown file
     * @param md
     *   Markdown file in a `String` format
+    * @param maybeRecoverOnError
+    *   function potentially recovering on errors
     * @return
     *   list of all found snippets
     */
-  def findCodeBlocks(md: String): Seq[MarkdownCodeBlock] = {
+  def findCodeBlocks(
+    subPath: os.SubPath,
+    md: String,
+    maybeRecoverOnError: BuildException => Option[BuildException]
+  ): Either[BuildException, Seq[MarkdownCodeBlock]] = {
     val allLines = md
       .lines()
       .toList
@@ -63,20 +73,24 @@ object MarkdownCodeBlock {
       closedFences: Seq[MarkdownCodeBlock] = Seq.empty,
       maybeOpenFence: Option[MarkdownOpenFence] = None,
       currentIndex: Int = 0
-    ): Seq[MarkdownCodeBlock] = if lines.isEmpty then closedFences
-    else {
-      val currentLine = lines.head
-      val (newClosedFences, newOpenFence) = maybeOpenFence match {
-        case None => closedFences -> MarkdownOpenFence.maybeFence(currentLine, currentIndex)
-        case mof @ Some(openFence) =>
-          val backticksStart = currentLine.indexOf(openFence.backticks)
-          if backticksStart == openFence.indent &&
-            currentLine.forall(c => c == '`' || c.isWhitespace)
-          then (closedFences :+ openFence.closeFence(currentIndex, allLines.toArray)) -> None
-          else closedFences                                                           -> mof
-      }
-      findCodeBlocksRec(lines.tail, newClosedFences, newOpenFence, currentIndex + 1)
+    ): Either[BuildException, Seq[MarkdownCodeBlock]] = lines -> maybeOpenFence match {
+      case (Seq(currentLine, tail*), mof) =>
+        val (newClosedFences, newOpenFence) = mof match {
+          case None => closedFences -> MarkdownOpenFence.maybeFence(currentLine, currentIndex)
+          case Some(openFence) =>
+            val backticksStart = currentLine.indexOf(openFence.backticks)
+            if backticksStart == openFence.indent &&
+              currentLine.forall(c => c == '`' || c.isWhitespace)
+            then (closedFences :+ openFence.closeFence(currentIndex, allLines.toArray)) -> None
+            else closedFences -> Some(openFence)
+        }
+        findCodeBlocksRec(tail, newClosedFences, newOpenFence, currentIndex + 1)
+      case (Nil, Some(openFence)) =>
+        maybeRecoverOnError(openFence.toUnclosedBackticksError(os.pwd / subPath))
+          .map(e => Left(e))
+          .getOrElse(Right(closedFences))
+      case _ => Right(closedFences)
     }
-    findCodeBlocksRec(allLines.toSeq).filter(!_.shouldIgnore)
+    findCodeBlocksRec(allLines.toSeq).map(_.filter(!_.shouldIgnore))
   }
 }

modules/build/src/main/scala/scala/build/internal/markdown/MarkdownCodeWrapper.scala

@@ -1,6 +1,8 @@
 package scala.build.internal.markdown
 
 import scala.annotation.tailrec
+import scala.build.EitherCps.{either, value}
+import scala.build.errors.BuildException
 import scala.build.internal.markdown.MarkdownCodeBlock
 import scala.build.internal.{AmmUtil, Name}
 
@@ -21,12 +23,13 @@ object MarkdownCodeWrapper {
     */
   def apply(
     subPath: os.SubPath,
-    content: String
-  ): (Option[String], Option[String], Option[String]) = {
+    content: String,
+    maybeRecoverOnError: BuildException => Option[BuildException] = b => Some(b)
+  ): Either[BuildException, (Option[String], Option[String], Option[String])] = either {
     val (pkg, wrapper) = AmmUtil.pathToPackageWrapper(subPath)
     val maybePkgString =
       if pkg.isEmpty then None else Some(s"package ${AmmUtil.encodeScalaSourcePath(pkg)}")
-    val allSnippets                      = MarkdownCodeBlock.findCodeBlocks(content)
+    val allSnippets = value(MarkdownCodeBlock.findCodeBlocks(subPath, content, maybeRecoverOnError))
     val (rawSnippets, processedSnippets) = allSnippets.partition(_.isRaw)
     val (testSnippets, mainSnippets)     = processedSnippets.partition(_.isTest)
     val wrapperName                      = s"${wrapper.raw}_md"

modules/build/src/main/scala/scala/build/internal/markdown/MarkdownOpenFence.scala

@@ -1,5 +1,8 @@
 package scala.build.internal.markdown
 
+import scala.build.Position
+import scala.build.errors.MarkdownUnclosedBackticksError
+
 /** Representation for an open code block in Markdown. (open meaning the closing backticks haven't
   * yet been parsed or they aren't at all present)
   *
@@ -41,6 +44,21 @@ case class MarkdownOpenFence(
       tickEndLine - 1 // ending backticks have to be placed below the snippet
     )
   }
+
+  /** Converts the [[MarkdownOpenFence]] into a [[MarkdownUnclosedBackticksError]]
+    *
+    * @param mdPath
+    *   path to the Markdown file
+    * @return
+    *   a [[MarkdownUnclosedBackticksError]]
+    */
+  def toUnclosedBackticksError(mdPath: os.Path): MarkdownUnclosedBackticksError = {
+    val startCoordinates = tickStartLine -> indent
+    val endCoordinates =
+      tickStartLine -> (indent + backticks.length)
+    val position = Position.File(Right(mdPath), startCoordinates, endCoordinates)
+    MarkdownUnclosedBackticksError(backticks, Seq(position))
+  }
 }
 
 object MarkdownOpenFence {

modules/build/src/main/scala/scala/build/preprocessing/MarkdownPreprocessor.scala

@@ -82,7 +82,8 @@ case object MarkdownPreprocessor extends Preprocessor {
           }
       }
 
-    val (mainScalaCode, rawScalaCode, testScalaCode) = MarkdownCodeWrapper(subPath, content)
+    val (mainScalaCode, rawScalaCode, testScalaCode) =
+      value(MarkdownCodeWrapper(subPath, content, maybeRecoverOnError))
 
     val maybeMainFile = value(preprocessSnippets(mainScalaCode, ".scala"))
     val maybeRawFile  = value(preprocessSnippets(rawScalaCode, ".raw.scala"))

modules/build/src/test/scala/scala/build/tests/markdown/MarkdownCodeWrapperTests.scala

@@ -4,6 +4,8 @@ import scala.build.internal.markdown.MarkdownCodeWrapper
 import com.eed3si9n.expecty.Expecty.expect
 import os.RelPath
 
+import scala.build.Position
+import scala.build.errors.{BuildException, MarkdownUnclosedBackticksError}
 import scala.build.internal.AmmUtil
 
 class MarkdownCodeWrapperTests extends munit.FunSuite {
@@ -21,7 +23,7 @@ class MarkdownCodeWrapperTests extends munit.FunSuite {
         |Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur.
         |Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
         |""".stripMargin
-    expect(MarkdownCodeWrapper(os.sub / "Example.md", markdown) == (None, None, None))
+    expect(MarkdownCodeWrapper(os.sub / "Example.md", markdown) == Right((None, None, None)))
   }
 
   test("a simple Scala snippet is correctly extracted from markdown") {
@@ -39,7 +41,7 @@ class MarkdownCodeWrapperTests extends munit.FunSuite {
         |println("Hello")
         |}}""".stripMargin
     val result = MarkdownCodeWrapper(os.sub / "Example.md", markdown)
-    expect(result == (Some(expectedScala), None, None))
+    expect(result == Right((Some(expectedScala), None, None)))
   }
 
   test("a raw Scala snippet is correctly extracted from markdown") {
@@ -61,7 +63,7 @@ class MarkdownCodeWrapperTests extends munit.FunSuite {
         |}
         |""".stripMargin
     val result = MarkdownCodeWrapper(os.sub / "Example.md", markdown)
-    expect(result == (None, Some(expectedScala), None))
+    expect(result == Right((None, Some(expectedScala), None)))
   }
 
   test("a test Scala snippet is correctly extracted from markdown") {
@@ -85,7 +87,7 @@ class MarkdownCodeWrapperTests extends munit.FunSuite {
         |}
         |""".stripMargin
     val result = MarkdownCodeWrapper(os.sub / "Example.md", markdown)
-    expect(result == (None, None, Some(expectedScala)))
+    expect(result == Right((None, None, Some(expectedScala))))
   }
 
   test("a Scala snippet is skipped when it's marked as `ignore` in markdown") {
@@ -96,6 +98,56 @@ class MarkdownCodeWrapperTests extends munit.FunSuite {
         |println("Hello")
         |```
         |""".stripMargin
-    expect(MarkdownCodeWrapper(os.sub / "Example.md", markdown) == (None, None, None))
+    expect(MarkdownCodeWrapper(os.sub / "Example.md", markdown) == Right((None, None, None)))
+  }
+
+  test("an unclosed snippet produces a build error") {
+    val markdown =
+      """# Some snippet
+        |
+        |```scala
+        |println("Hello")
+        |""".stripMargin
+    val subPath          = os.sub / "Example.md"
+    val expectedPosition = Position.File(Right(os.pwd / subPath), 2 -> 0, 2 -> 3)
+    val expectedError    = MarkdownUnclosedBackticksError("```", Seq(expectedPosition))
+    val Left(result)     = MarkdownCodeWrapper(subPath, markdown)
+    expect(result.message == expectedError.message)
+    expect(result.positions == expectedError.positions)
+  }
+
+  test("recovery from an unclosed snippet error works correctly") {
+    val markdown =
+      """# Some snippet
+        |```scala
+        |println("closed snippet")
+        |```
+        |
+        |# Some other snippet
+        |
+        |````scala
+        |println("unclosed snippet")
+        |
+        |```scala
+        |println("whatever")
+        |```
+        |""".stripMargin
+    val subPath                             = os.sub / "Example.md"
+    var actualError: Option[BuildException] = None
+    val recoveryFunction = (be: BuildException) => {
+      actualError = Some(be)
+      None
+    }
+    val Right(result) =
+      MarkdownCodeWrapper(subPath, markdown, maybeRecoverOnError = recoveryFunction)
+    val expectedScala =
+      """object Example_md { @annotation.nowarn("msg=pure expression does nothing") def main(args: Array[String]): Unit = { Scope; }
+        |object Scope {
+        |println("closed snippet")
+        |}}""".stripMargin
+    expect(result == (Some(expectedScala), None, None))
+    val expectedPosition = Position.File(Right(os.pwd / subPath), 7 -> 0, 7 -> 4)
+    val expectedError    = MarkdownUnclosedBackticksError("````", Seq(expectedPosition))
+    expect(actualError.get.positions == expectedError.positions)
   }
 }

modules/core/src/main/scala/scala/build/errors/MarkdownUnclosedBackticksError.scala

@@ -0,0 +1,12 @@
+package scala.build.errors
+import scala.build.Position
+
+class MarkdownUnclosedBackticksError(
+  backticks: String,
+  positions: Seq[Position]
+) extends BuildException(s"Unclosed $backticks code block in a Markdown input", positions)
+
+object MarkdownUnclosedBackticksError {
+  def apply(backticks: String, positions: Seq[Position]) =
+    new MarkdownUnclosedBackticksError(backticks, positions)
+}