Fix negative parallelism and negative semaphore (#23902)

* Throw an error when parallelism <=0

* Panic in case of negative semaphore

terraform/context.go

@@ -138,7 +138,17 @@ func NewContext(opts *ContextOpts) (*Context, tfdiags.Diagnostics) {
 	// Determine parallelism, default to 10. We do this both to limit
 	// CPU pressure but also to have an extra guard against rate throttling
 	// from providers.
+	// We throw an error in case of negative parallelism
 	par := opts.Parallelism
+	if par < 0 {
+		diags = diags.Append(tfdiags.Sourceless(
+			tfdiags.Error,
+			"Invalid parallelism value",
+			fmt.Sprintf("The parallelism must be a positive value. Not %d.", par),
+		))
+		return nil, diags
+	}
+
 	if par == 0 {
 		par = 10
 	}

terraform/util.go

@@ -12,8 +12,8 @@ type Semaphore chan struct{}
 // NewSemaphore creates a semaphore that allows up
 // to a given limit of simultaneous acquisitions
 func NewSemaphore(n int) Semaphore {
-	if n == 0 {
-		panic("semaphore with limit 0")
+	if n <= 0 {
+		panic("semaphore with limit <=0")
 	}
 	ch := make(chan struct{}, n)
 	return Semaphore(ch)