add code to prevent segfault during Eval of thunk Expr

[ssikdar1]

Description

Trying to address issue #32315.

I have a feeling this isn't correct, but trying to code against the situation where a :thunk Expr is passed to Eval with incorrect values.

For example the following now would have an error returned where they previously segfaulted.

julia> Main.eval(Expr(:thunk, x->x+9))
ERROR: syntax: malformed "thunk" statement
Stacktrace:
 [1] top-level scope at none:1
 [2] eval at ./boot.jl:331 [inlined]
 [3] eval(::Expr) at ./client.jl:452
 [4] top-level scope at REPL[1]:1

julia> Main.eval(Expr(:thunk, Meta.parse("x=17")))
ERROR: null stmt in body_attributes
Stacktrace:
 [1] eval at ./boot.jl:331 [inlined]
 [2] eval(::Expr) at ./client.jl:452
 [3] top-level scope at REPL[2]:1

Although the following does still segfault:

julia> Main.eval(Expr(:thunk, Meta.parse("17")))

signal (11): Segmentation fault: 11
in expression starting at none:1
body_attributes at /Users/ssikdar1/julia/src/toplevel.c:369 [inlined]
jl_toplevel_eval_flex at /Users/ssikdar1/julia/src/toplevel.c:801
jl_toplevel_eval at /Users/ssikdar1/julia/src/toplevel.c:834 [inlined]
jl_toplevel_eval_in at /Users/ssikdar1/julia/src/toplevel.c:857
eval at ./boot.jl:331 [inlined]
eval at ./client.jl:452
jl_apply at /Users/ssikdar1/julia/src/./julia.h:1691 [inlined]
do_call at /Users/ssikdar1/julia/src/interpreter.c:369
eval_stmt_value at /Users/ssikdar1/julia/src/interpreter.c:409 [inlined]

Closes #32315

[JeffBezanson]

Yes, I don't think this is correct because it doesn't actually check the structure. It checks much later, when having the wrong structure might have resulted in some null pointers, but only if you're lucky.

[JeffBezanson]

I think it would be enough to turn this assert into an error.

[ssikdar1]

Changed the assert to error in a7c56d7

[ssikdar1]

@JeffBezanson Changed the assert to error in a7c56d7.

Now all 3 of the examples no longer segfault.

julia> Main.eval(Expr(:thunk, Meta.parse("17")))
ERROR: malformed "thunk" statement
Stacktrace:
 [1] top-level scope at none:1
 [2] eval at ./boot.jl:331 [inlined]
 [3] eval(::Expr) at ./client.jl:452
 [4] top-level scope at REPL[1]:1

julia> Main.eval(Expr(:thunk, Meta.parse("x=17")))
ERROR: malformed "thunk" statement
Stacktrace:
 [1] top-level scope at none:1
 [2] eval at ./boot.jl:331 [inlined]
 [3] eval(::Expr) at ./client.jl:452
 [4] top-level scope at REPL[1]:1

julia> Main.eval(Expr(:thunk, x->x+9))
ERROR: malformed "thunk" statement
Stacktrace:
 [1] top-level scope at none:1
 [2] eval at ./boot.jl:331 [inlined]
 [3] eval(::Expr) at ./client.jl:452
 [4] top-level scope at REPL[2]:1