SAN FRANCISCO — The Giants have one of the worst bullpens in the major leagues. That late-inning leakiness helps explain how they began the week as the only MLB team that hadn’t swept a three-game series this season. They’d lost eight games in which they led in the ninth inning.Even the leakiest bullpen should be able to skip over a puddle, though. The Giants held a six-run lead entering the ninth inning Wednesday afternoon. They were three outs away from polishing off a victory over the Cincinnati Reds and, at long last, a three-game sweep.Instead, they sank, in historic fashion.The Reds sent the first nine batters to the plate against right-handers Spencer Bivens and Dylan Smith and the only out the Giants recorded was a sacrifice fly. Bivens allowed two hits and two walks; Eugenio Suárez greeted Smith with a grand slam, and a misplayed bunt that the entire ballpark saw coming set up the Reds to score the tying and go-ahead runs. By the time the carnage was over, a 9-3 lead had turned into a one-run deficit. Boos cascaded from the stands when the Giants went down meekly in the ninth to seal a 10-9 loss on the shores of McCovey Cove.Who should win NL MVP?Joe CrisalliIt was the largest blown lead in the ninth inning in Giants history. It was the Reds’ largest ninth-inning comeback victory since May 8, 1958, when they overcame a six-run deficit to beat the Chicago Cubs at Wrigley Field.Even for a Giants team accustomed to disappointment, Wednesday’s result left them dumbstruck. They had been 2,208-0 all-time when leading by six or more runs in the ninth inning.Giants manager Tony Vitello, who was ejected just five batters into the first inning for arguing a checked swing, watched the game on two television feeds in the clubhouse. As if one view wasn’t enough.“Don’t really have a word for it,” Vitello said. “More than unfortunate.”Giants baseball: More than unfortunate. There’s your slogan for the season. There is no redefining a lost year at this late stage. There is no hope that this team will turn a corner. By this point, the best they can hope to achieve is five minutes when they feel good about themselves. And after Logan Webb moved himself up in the rotation, and Rafael Devers hoisted an otherwise anonymous lineup while hitting a home run for the third consecutive game, that’s what they were about to do.Then Bivens, who replaced injured right-hander Adrian Houser on the roster Wednesday morning, went homer, single, wild pitch, walk, walk. Smith served up the slam to Suárez on a high fastball. Then the Giants’ inexperienced infield had trouble converting bunts into outs. Dane Myers tried to bunt for a hit and failed to deaden the ball, but third baseman Buddy Kennedy skipped a throw that Devers wasn’t able to scoop. With Myers at second base, Matt McLain bunted the tying run to third and reached safely when he beat Smith’s throw. After McLain stole second base, Jose Trevino’s sacrifice fly drew the Reds even and Tyler Stephenson lined a pinch single to left field to give them the lead.Smith has been walked off in four ballparks this season. Becoming a part of the worst ninth-inning meltdown in Giants history had to feel even worse.