ircd::db: Enrich seek(row) opts; add proper error handling and propagation.

include/ircd/db/opts.h

@@ -50,6 +50,9 @@ enum class ircd::db::get
 	NO_CHECKSUM      = 0x0020, // Integrity of data will not be checked (overrides conf).
 	PREFIX           = 0x0040, // (prefix_same_as_start); automatic for index columns with pfx
 	ORDERED          = 0x0080, // (total_order_seek); relevant to index columns
+	NO_PARALLEL      = 0x0100, // Don't submit requests in parallel (relevant to db::row)
+	NO_THROW         = 0x0200, // Suppress exceptions if possible.
+	THROW            = 0x0400, // Throw exceptions more than usual.
 };
 
 template<class T>

include/ircd/db/row.h

@@ -60,7 +60,7 @@ struct ircd::db::row
 	    gopts opts = {})
 	__attribute__((stack_protect));
 
-	friend size_t seek(row &, const string_view &);
+	friend size_t seek(row &, const string_view &, const gopts &opts = {});
 };
 
 namespace ircd::db

ircd/db.cc

@@ -9005,58 +9005,104 @@ ircd::db::write(const row::delta *const &begin,
 
 size_t
 ircd::db::seek(row &r,
-               const string_view &key)
+               const string_view &key,
+               const gopts &opts)
 {
-	// This frame can't be interrupted because it may have requests
-	// pending in the request pool which must synchronize back here.
-	const ctx::uninterruptible ui;
-
 	#ifdef RB_DEBUG_DB_SEEK
 	const ircd::timer timer;
 	#endif
 
+	// This frame can't be interrupted because it may have requests
+	// pending in the request pool which must synchronize back here.
+	const ctx::uninterruptible ui;
+
 	size_t ret{0};
+	std::exception_ptr eptr;
 	ctx::latch latch{r.size()};
-	const auto closure{[&latch, &ret, &key]
-	(auto &cell)
+	const auto closure{[&opts, &latch, &ret, &key, &eptr]
+	(auto &cell) noexcept
 	{
-		ret += bool(seek(cell, key));
+		// If there's a pending error from another cell by the time this
+		// closure is executed we don't perform the seek() unless the user
+		// specifies db::get::NO_THROW to suppress it.
+		if(!eptr || test(opts, get::NO_THROW)) try
+		{
+			if(!seek(cell, key))
+			{
+				// If the cell is not_found that's not a thrown exception here;
+				// the cell will just be !valid(). The user can specify
+				// get::THROW to propagate a not_found from the seek(row);
+				if(test(opts, get::THROW))
+					throw not_found
+					{
+						"column '%s' key '%s'", cell.col(), key
+					};
+			}
+			else ++ret;
+		}
+		catch(const not_found &e)
+		{
+			eptr = std::current_exception();
+		}
+		catch(const std::exception &e)
+		{
+			log::error
+			{
+				log, "row seek: column '%s' key '%s' :%s",
+				cell.col(),
+				key,
+				e.what()
+			};
+
+			eptr = std::make_exception_ptr(e);
+		}
+
+		// The latch must always be hit here. No exception should propagate
+		// to prevent this from being reached or beyond.
 		latch.count_down();
 	}};
 
+	// Submit all the requests
 	for(auto &cell : r)
 	{
 		db::column &column(cell);
-		//TODO: should check a bloom filter on the cache for this branch
-		//TODO: because right now double-querying the cache is gross.
-		if(!exists(cache(column), key))
-			request([&closure, &cell]
+		const auto reclosure{[&closure, &cell]
 		{
 			closure(cell);
-			});
+		}};
+
+		//TODO: should check a bloom filter on the cache for this branch
+		//TODO: because right now double-querying the cache is gross.
+		if(!test(opts, get::NO_PARALLEL) && !exists(cache(column), key))
+			request(reclosure);
 		else
-			closure(cell);
+			reclosure();
 	}
 
+	// Wait for responses.
 	latch.wait();
+	assert(ret <= r.size());
 
 	#ifdef RB_DEBUG_DB_SEEK
 	const column &c(r[0]);
 	const database &d(c);
 	log::debug
 	{
-		log, "'%s' %lu:%lu '%s' row SEEK KEY %zu of %zu in %ld$us",
+		log, "'%s' %lu:%lu '%s' row SEEK KEY %zu of %zu in %ld$us %s",
 		name(d),
 		sequence(d),
 		sequence(r[0]),
 		name(c),
 		ret,
 		r.size(),
-		timer.at<microseconds>().count()
+		timer.at<microseconds>().count(),
+		what(eptr)
 	};
 	#endif
 
-	assert(ret <= r.size());
+	if(eptr && !test(opts, get::NO_THROW))
+		std::rethrow_exception(eptr);
+
 	return ret;
 }
 
@@ -9144,7 +9190,7 @@ ircd::db::row::row(database &d,
 	}
 
 	if(key)
-		seek(*this, key);
+		seek(*this, key, opts);
 }
 #pragma GCC diagnostic pop