Lindsay Lohan -- Sober Coachella ... Mission Accomplished

Lindsay Lohan stuck to her guns this weekend at Coachella ... telling TMZ, she followed through on her promise NOT to take a sip of booze the entire weekend -- AND WHY WOULD SHE LIE????

Lindsay was out in Malibu earlier today when we asked if she stayed clean as a whistle at the booze-filled drug-infested music festival?

We also asked about her favorite Coachella act ... and what her favorite non-alcoholic drink is? She actually answers.