[lldb] Eliminate SupportFileSP nullptr derefs (#168624)

This patch fixes and eliminates the possibility of SupportFileSP ever
being nullptr. The support file was originally treated like a value
type, but became a polymorphic type and therefore has to be stored and
passed around as a pointer.

To avoid having all the callers check the validity of the pointer, I
introduced the invariant that SupportFileSP is never null and always
default constructed. However, without enforcement at the type level,
that's fragile and indeed, we already identified two crashes where
someone accidentally broke that invariant.

This PR introduces a NonNullSharedPtr to prevent that. NonNullSharedPtr
is a smart pointer wrapper around std::shared_ptr that guarantees the
pointer is never null. If default-constructed, it creates a
default-constructed instance of the contained type. Note that I'm using
private inheritance because you shouldn't inherit from standard library
classes due to the lack of virtual destructor. So while the new
abstraction looks like a `std::shared_ptr`, it is in fact **not** a
shared pointer. Given that our destructor is trivial, we could use
public inheritance, but currently there's no need for it.

rdar://164989579

1 files changed, 1 insertions, 1 deletions

diff --git a/lldb/source/Breakpoint/BreakpointResolverFileLine.cpp b/lldb/source/Breakpoint/BreakpointResolverFileLine.cpp
index a94e9e23163d..cef1ef1b08fe 100644
--- a/lldb/source/Breakpoint/BreakpointResolverFileLine.cpp
+++ b/lldb/source/Breakpoint/BreakpointResolverFileLine.cpp
@@ -139,7 +139,7 @@ void BreakpointResolverFileLine::FilterContexts(SymbolContextList &sc_list) {
     if (!sc.block)
       continue;
 
-    SupportFileSP file_sp;
+    SupportFileNSP file_sp = std::make_shared<SupportFile>();
     uint32_t line;
     const Block *inline_block = sc.block->GetContainingInlinedBlock();
     if (inline_block) {