20812: Add functions for exception handling

Functions/Exceptions/catch

@@ -0,0 +1,41 @@
+# Catch an exception.  Returns 0 if the exception in question was caught.
+# The first argument gives the exception to catch, which may be a
+# pattern.
+# This must be within an always-block.  A typical set of handlers looks
+# like:
+#   {
+#     # try block; something here throws exceptions
+#   } always {
+#      if catch MyExcept; then
+#         # Handler code goes here.
+#         print Handling exception MyExcept
+#      elif catch *; then
+#         # This is the way to implement a catch-all.
+#         print Handling any other exception
+#      fi
+#   }
+# As with other languages, exceptions do not need to be handled
+# within an always block and may propagate to a handler further up the
+# call chain.
+#
+# It is possible to throw an exception from within the handler by
+# using "throw".
+#
+# The shell variable $CAUGHT is set to the last exception caught,
+# which is useful if the argument to "catch" was a pattern.
+#
+# Use "function" keyword in case catch is already an alias.
+function catch {
+  if [[ $TRY_BLOCK_ERROR -gt 0 && $EXCEPTION = ${~1} ]]; then
+    (( TRY_BLOCK_ERROR = 0 ))
+    CAUGHT="$EXCEPTION"
+    unset EXCEPTION
+    return 0
+  fi
+
+  return 1
+}
+# Never use globbing with "catch".
+alias catch="noglob catch"
+
+catch "$@"

Functions/Exceptions/throw

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+# 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