Ensure rows are only valid while currently being iterated (#262)

Fixes #251 amongst other issues. This PR makes `SQLite.Row` explicitly
error when values are attempted to be accessed and it's not the
currently iterated row. It borrows the idea from the MySQL.jl package,
which as a similar "sync" of row numbers between the `Row` and `Query`
objects.

Author: quinnj

src/tables.jl

@@ -8,13 +8,15 @@ struct Query
     names::Vector{Symbol}
     types::Vector{Type}
     lookup::Dict{Symbol, Int}
+    current_rownumber::Base.RefValue{Int}
 end
 
 # check if the query has no (more) rows
 Base.isempty(q::Query) = q.status[] == SQLITE_DONE
 
 struct Row <: Tables.AbstractRow
     q::Query
+    rownumber::Int
 end
 
 getquery(r::Row) = getfield(r, :q)
@@ -56,7 +58,10 @@ function done(q::Query)
     return false
 end
 
-function getvalue(q::Query, col::Int, ::Type{T}) where {T}
+@noinline wrongrow(i) = throw(ArgumentError("row $i is no longer valid; sqlite query results are forward-only iterators where each row is only valid when iterated; re-execute the query, convert rows to NamedTuples, or stream the results to a sink to save results"))
+
+function getvalue(q::Query, col::Int, rownumber::Int, ::Type{T}) where {T}
+    rownumber == q.current_rownumber[] || wrongrow(rownumber)
     handle = _stmt(q.stmt).handle
     t = sqlite3_column_type(handle, col)
     if t == SQLITE_NULL
@@ -67,21 +72,23 @@ function getvalue(q::Query, col::Int, ::Type{T}) where {T}
     end
 end
 
-Tables.getcolumn(r::Row, ::Type{T}, i::Int, nm::Symbol) where {T} = getvalue(getquery(r), i, T)
+Tables.getcolumn(r::Row, ::Type{T}, i::Int, nm::Symbol) where {T} = getvalue(getquery(r), i, getfield(r, :rownumber), T)
 
 Tables.getcolumn(r::Row, i::Int) = Tables.getcolumn(r, getquery(r).types[i], i, getquery(r).names[i])
 Tables.getcolumn(r::Row, nm::Symbol) = Tables.getcolumn(r, getquery(r).lookup[nm])
 Tables.columnnames(r::Row) = Tables.columnnames(getquery(r))
 
 function Base.iterate(q::Query)
     done(q) && return nothing
-    return Row(q), nothing
+    q.current_rownumber[] = 1
+    return Row(q, 1), 2
 end
 
-function Base.iterate(q::Query, ::Nothing)
+function Base.iterate(q::Query, rownumber)
     q.status[] = sqlite3_step(_stmt(q.stmt).handle)
     done(q) && return nothing
-    return Row(q), nothing
+    q.current_rownumber[] = rownumber
+    return Row(q, rownumber), rownumber + 1
 end
 
 "Return the last row insert id from the executed statement"
@@ -129,7 +136,7 @@ function DBInterface.execute(stmt::Stmt, params::DBInterface.StatementParams; al
         header[i] = nm
         types[i] = Union{juliatype(_st.handle, i), Missing}
     end
-    return Query(stmt, Ref(status), header, types, Dict(x=>i for (i, x) in enumerate(header)))
+    return Query(stmt, Ref(status), header, types, Dict(x=>i for (i, x) in enumerate(header)), Ref(0))
 end
 
 """

test/runtests.jl

@@ -540,4 +540,11 @@ tbl = DBInterface.execute(db, "select * from tbl") |> columntable
     c = [3, 3, 3]
 )
 
+# https://github.com/JuliaDatabases/SQLite.jl/issues/251
+q = DBInterface.execute(db, "select * from tbl")
+row, st = iterate(q)
+@test row.a == 1 && row.b == 2 && row.c == 3
+row2, st = iterate(q, st)
+@test_throws ArgumentError row.a
+
 end