Fix integer overflow in skip(s::IOBuffer, typemax(Int64)) (#54070)

Fixes #53908  by clamping before doing addition.

This also fixes an issue with negative skips if `io.offset` isn't zero.

I am assuming that `io.size+1` cannot overflow.

(cherry picked from commit 306124c)

base/iobuffer.jl

@@ -260,22 +260,32 @@ bytesavailable(io::GenericIOBuffer) = io.size - io.ptr + 1
 position(io::GenericIOBuffer) = io.ptr - io.offset - 1
 
 function skip(io::GenericIOBuffer, n::Integer)
-    seekto = io.ptr + n
-    n < 0 && return seek(io, seekto-1) # Does error checking
-    io.ptr = min(seekto, io.size+1)
-    return io
+    skip(io, clamp(n, Int))
+end
+function skip(io::GenericIOBuffer, n::Int)
+    if signbit(n)
+        seekto = clamp(widen(position(io)) + widen(n), Int)
+        seek(io, seekto) # Does error checking
+    else
+        n_max = io.size + 1 - io.ptr
+        io.ptr += min(n, n_max)
+        io
+    end
 end
 
 function seek(io::GenericIOBuffer, n::Integer)
+    seek(io, clamp(n, Int))
+end
+function seek(io::GenericIOBuffer, n::Int)
     if !io.seekable
         ismarked(io) || throw(ArgumentError("seek failed, IOBuffer is not seekable and is not marked"))
         n == io.mark || throw(ArgumentError("seek failed, IOBuffer is not seekable and n != mark"))
     end
     # TODO: REPL.jl relies on the fact that this does not throw (by seeking past the beginning or end
     #       of an GenericIOBuffer), so that would need to be fixed in order to throw an error here
-    #(n < 0 || n > io.size) && throw(ArgumentError("Attempted to seek outside IOBuffer boundaries."))
-    #io.ptr = n+1
-    io.ptr = min(max(0, n)+io.offset, io.size)+1
+    #(n < 0 || n > io.size - io.offset) && throw(ArgumentError("Attempted to seek outside IOBuffer boundaries."))
+    #io.ptr = n + io.offset + 1
+    io.ptr = clamp(n, 0, io.size - io.offset) + io.offset + 1
     return io
 end
 

test/iobuffer.jl

@@ -196,6 +196,31 @@ end
     @test position(skip(io, -3)) == 0
 end
 
+@testset "issue #53908" begin
+    @testset "offset $first" for first in (false, true)
+        b = collect(0x01:0x05)
+        sizehint!(b, 100; first) # make offset non zero
+        io = IOBuffer(b)
+        @test position(skip(io, 4)) == 4
+        @test position(skip(io, typemax(Int))) == 5
+        @test position(skip(io, typemax(Int128))) == 5
+        @test position(skip(io, typemax(Int32))) == 5
+        @test position(skip(io, typemin(Int))) == 0
+        @test position(skip(io, typemin(Int128))) == 0
+        @test position(skip(io, typemin(Int32))) == 0
+        @test position(skip(io, 4)) == 4
+        @test position(skip(io, -2)) == 2
+        @test position(skip(io, -2)) == 0
+        @test position(seek(io, -2)) == 0
+        @test position(seek(io, typemax(Int))) == 5
+        @test position(seek(io, typemax(Int128))) == 5
+        @test position(seek(io, typemax(Int32))) == 5
+        @test position(seek(io, typemin(Int))) == 0
+        @test position(seek(io, typemin(Int128))) == 0
+        @test position(seek(io, typemin(Int32))) == 0
+    end
+end
+
 @testset "pr #11554" begin
     io  = IOBuffer(SubString("***αhelloworldω***", 4, 16))
     io2 = IOBuffer(Vector{UInt8}(b"goodnightmoon"), read=true, write=true)