20812: Add functions for exception handling

Functions/Exceptions/throw

@@ -0,0 +1,29 @@
+# Throw an exception.
+# The first argument is a string giving the exception.  Other arguments
+# are ignored.
+#
+# This is designed to be called somewhere inside a "try-block", i.e.
+# some code of the form:
+#   {
+#     # try-block
+#   } always {
+#     # always-block
+#   }
+# although as normal with exceptions it might be hidden deep inside
+# other code.  Note, however, that it must be code running within the
+# current shell; with shells, unlike other languages, it is quite easy
+# to miss points at which the shell forks.
+#
+# If there is nothing to catch an exception, this behaves like any
+# other shell error, aborting to the command prompt or abandoning a
+# script.
+
+# The following must not be local.
+EXCEPTION="$1"
+if (( TRY_BLOCK_ERROR == 0 )); then
+  # We are throwing an exception from the middle of an always-block.
+  # We can do this by restoring the error status from the try-block.
+  (( TRY_BLOCK_ERROR = 1 ))
+fi
+# Raise an error, but don't show an error message.
+{ ${:?THROW} } 2>/dev/null