Create infrastructure for "soft" error reporting.

Postgres' standard mechanism for reporting errors (ereport() or elog())
is used for all sorts of error conditions.  This means that throwing
an exception via ereport(ERROR) requires an expensive transaction or
subtransaction abort and cleanup, since the exception catcher dare not
make many assumptions about what has gone wrong.  There are situations
where we would rather have a lighter-weight mechanism for dealing
with errors that are known to be safe to recover from without a full
transaction cleanup.  This commit creates infrastructure to let us
adapt existing error-reporting code for that purpose.  See the
included documentation changes for details.  Follow-on commits will
provide test code and usage examples.

The near-term plan is to convert most if not all datatype input
functions to report invalid input "softly".  This will enable
implementing some SQL/JSON features cleanly and without the cost
of subtransactions, and it will also allow creating COPY options
to deal with bad input without cancelling the whole COPY.

This patch is mostly by me, but it owes very substantial debt to
earlier work by Nikita Glukhov, Andrew Dunstan, and Amul Sul.
Thanks also to Andres Freund for review.

Discussion: https://postgr.es/m/3bbbb0df-7382-bf87-9737-340ba096e034@postgrespro.ru

1 files changed, 123 insertions, 0 deletions

diff --git a/src/backend/utils/error/elog.c b/src/backend/utils/error/elog.c
index f5cd1b74937..eb489ea3a70 100644
--- a/src/backend/utils/error/elog.c
+++ b/src/backend/utils/error/elog.c
@@ -71,6 +71,7 @@
 #include "libpq/libpq.h"
 #include "libpq/pqformat.h"
 #include "mb/pg_wchar.h"
+#include "nodes/miscnodes.h"
 #include "miscadmin.h"
 #include "pgstat.h"
 #include "postmaster/bgworker.h"
@@ -611,6 +612,128 @@ errfinish(const char *filename, int lineno, const char *funcname)
 	CHECK_FOR_INTERRUPTS();
 }
 
+
+/*
+ * errsave_start --- begin a "soft" error-reporting cycle
+ *
+ * If "context" isn't an ErrorSaveContext node, this behaves as
+ * errstart(ERROR, domain), and the errsave() macro ends up acting
+ * exactly like ereport(ERROR, ...).
+ *
+ * If "context" is an ErrorSaveContext node, but the node creator only wants
+ * notification of the fact of a soft error without any details, we just set
+ * the error_occurred flag in the ErrorSaveContext node and return false,
+ * which will cause us to skip the remaining error processing steps.
+ *
+ * Otherwise, create and initialize error stack entry and return true.
+ * Subsequently, errmsg() and perhaps other routines will be called to further
+ * populate the stack entry.  Finally, errsave_finish() will be called to
+ * tidy up.
+ */
+bool
+errsave_start(struct Node *context, const char *domain)
+{
+	ErrorSaveContext *escontext;
+	ErrorData  *edata;
+
+	/*
+	 * Do we have a context for soft error reporting?  If not, just punt to
+	 * errstart().
+	 */
+	if (context == NULL || !IsA(context, ErrorSaveContext))
+		return errstart(ERROR, domain);
+
+	/* Report that a soft error was detected */
+	escontext = (ErrorSaveContext *) context;
+	escontext->error_occurred = true;
+
+	/* Nothing else to do if caller wants no further details */
+	if (!escontext->details_wanted)
+		return false;
+
+	/*
+	 * Okay, crank up a stack entry to store the info in.
+	 */
+
+	recursion_depth++;
+
+	/* Initialize data for this error frame */
+	edata = get_error_stack_entry();
+	edata->elevel = LOG;		/* signal all is well to errsave_finish */
+	set_stack_entry_domain(edata, domain);
+	/* Select default errcode based on the assumed elevel of ERROR */
+	edata->sqlerrcode = ERRCODE_INTERNAL_ERROR;
+
+	/*
+	 * Any allocations for this error state level should go into the caller's
+	 * context.  We don't need to pollute ErrorContext, or even require it to
+	 * exist, in this code path.
+	 */
+	edata->assoc_context = CurrentMemoryContext;
+
+	recursion_depth--;
+	return true;
+}
+
+/*
+ * errsave_finish --- end a "soft" error-reporting cycle
+ *
+ * If errsave_start() decided this was a regular error, behave as
+ * errfinish().  Otherwise, package up the error details and save
+ * them in the ErrorSaveContext node.
+ */
+void
+errsave_finish(struct Node *context, const char *filename, int lineno,
+			   const char *funcname)
+{
+	ErrorSaveContext *escontext = (ErrorSaveContext *) context;
+	ErrorData  *edata = &errordata[errordata_stack_depth];
+
+	/* verify stack depth before accessing *edata */
+	CHECK_STACK_DEPTH();
+
+	/*
+	 * If errsave_start punted to errstart, then elevel will be ERROR or
+	 * perhaps even PANIC.  Punt likewise to errfinish.
+	 */
+	if (edata->elevel >= ERROR)
+	{
+		errfinish(filename, lineno, funcname);
+		pg_unreachable();
+	}
+
+	/*
+	 * Else, we should package up the stack entry contents and deliver them to
+	 * the caller.
+	 */
+	recursion_depth++;
+
+	/* Save the last few bits of error state into the stack entry */
+	set_stack_entry_location(edata, filename, lineno, funcname);
+
+	/* Replace the LOG value that errsave_start inserted */
+	edata->elevel = ERROR;
+
+	/*
+	 * We skip calling backtrace and context functions, which are more likely
+	 * to cause trouble than provide useful context; they might act on the
+	 * assumption that a transaction abort is about to occur.
+	 */
+
+	/*
+	 * Make a copy of the error info for the caller.  All the subsidiary
+	 * strings are already in the caller's context, so it's sufficient to
+	 * flat-copy the stack entry.
+	 */
+	escontext->error_data = palloc_object(ErrorData);
+	memcpy(escontext->error_data, edata, sizeof(ErrorData));
+
+	/* Exit error-handling context */
+	errordata_stack_depth--;
+	recursion_depth--;
+}
+
+
 /*
  * get_error_stack_entry --- allocate and initialize a new stack entry
  *