From 2dfff15a9dfbe0d21d9803cb0753f9dd8ea35608 Mon Sep 17 00:00:00 2001 From: rw Date: Wed, 11 Nov 2015 17:08:16 -0800 Subject: [PATCH] Improve Builder user interface. + Add state to the Builder object to track if we are inside a table, and if we are finished building the buffer. + Use this data to check that a buffer is being built correctly. + Panic if a buffer is not being built correctly. + Test that the panics happen as expected. Based on d236dea13d2fdb9ad596679868eb4204c1562151. --- go/builder.go | 102 +++++++++++++++++++++++++++++++++-------------- tests/go_test.go | 30 +++++++------- 2 files changed, 88 insertions(+), 44 deletions(-) diff --git a/go/builder.go b/go/builder.go index 7533dddf2..cf21dd55d 100644 --- a/go/builder.go +++ b/go/builder.go @@ -6,14 +6,17 @@ package flatbuffers // A Builder constructs byte buffers in a last-first manner for simplicity and // performance. type Builder struct { + // `Bytes` gives raw access to the buffer. Most users will want to use + // FinishedBytes() instead. Bytes []byte - minalign int - vtable []UOffsetT - objectEnd UOffsetT - insideObject bool - vtables []UOffsetT - head UOffsetT + minalign int + vtable []UOffsetT + objectEnd UOffsetT + vtables []UOffsetT + head UOffsetT + nested bool + finished bool } // NewBuilder initializes a Builder of size `initial_size`. @@ -33,7 +36,7 @@ func NewBuilder(initialSize int) *Builder { } // Reset truncates the underlying Builder buffer, facilitating alloc-free -// reuse of a Builder. +// reuse of a Builder. It also resets bookkeeping data. func (b *Builder) Reset() { if b.Bytes != nil { b.Bytes = b.Bytes[:cap(b.Bytes)] @@ -49,12 +52,22 @@ func (b *Builder) Reset() { b.head = UOffsetT(len(b.Bytes)) b.minalign = 1 + b.nested = false + b.finished = false +} + +// FinishedBytes returns a pointer to the written data in the byte buffer. +// Panics if the builder is not in a finished state (which is caused by calling +// `Finish()`). +func (b *Builder) FinishedBytes() []byte { + b.assertFinished() + return b.Bytes[b.Head():] } // StartObject initializes bookkeeping for writing a new object. func (b *Builder) StartObject(numfields int) { - b.notNested() - b.insideObject = true + b.assertNotNested() + b.nested = true // use 32-bit offsets so that arithmetic doesn't overflow. if cap(b.vtable) < numfields || b.vtable == nil { @@ -129,7 +142,7 @@ func (b *Builder) WriteVtable() (n UOffsetT) { var off UOffsetT if b.vtable[i] != 0 { // Forward reference to field; - // use 32bit number to ensure no overflow: + // use 32bit number to assert no overflow: off = objectOffset - b.vtable[i] } @@ -173,11 +186,9 @@ func (b *Builder) WriteVtable() (n UOffsetT) { // EndObject writes data necessary to finish object construction. func (b *Builder) EndObject() UOffsetT { - if !b.insideObject { - panic("not in object") - } + b.assertNested() n := b.WriteVtable() - b.insideObject = false + b.nested = false return n } @@ -271,7 +282,8 @@ func (b *Builder) PrependUOffsetT(off UOffsetT) { // // +, where T is the type of elements of this vector. func (b *Builder) StartVector(elemSize, numElems, alignment int) UOffsetT { - b.notNested() + b.assertNotNested() + b.nested = true b.Prep(SizeUint32, elemSize*numElems) b.Prep(alignment, elemSize*numElems) // Just in case alignment > int. return b.Offset() @@ -279,14 +291,19 @@ func (b *Builder) StartVector(elemSize, numElems, alignment int) UOffsetT { // EndVector writes data necessary to finish vector construction. func (b *Builder) EndVector(vectorNumElems int) UOffsetT { + b.assertNested() + // we already made space for this, so write without PrependUint32 b.PlaceUOffsetT(UOffsetT(vectorNumElems)) + + b.nested = false return b.Offset() } // CreateString writes a null-terminated string as a vector. func (b *Builder) CreateString(s string) UOffsetT { - b.notNested() + b.assertNotNested() + b.nested = true b.Prep(int(SizeUOffsetT), (len(s)+1)*SizeByte) b.PlaceByte(0) @@ -301,7 +318,8 @@ func (b *Builder) CreateString(s string) UOffsetT { // CreateByteString writes a byte slice as a string (null-terminated). func (b *Builder) CreateByteString(s []byte) UOffsetT { - b.notNested() + b.assertNotNested() + b.nested = true b.Prep(int(SizeUOffsetT), (len(s)+1)*SizeByte) b.PlaceByte(0) @@ -316,6 +334,9 @@ func (b *Builder) CreateByteString(s []byte) UOffsetT { // CreateByteVector writes a ubyte vector func (b *Builder) CreateByteVector(v []byte) UOffsetT { + b.assertNotNested() + b.nested = true + b.Prep(int(SizeUOffsetT), len(v)*SizeByte) l := UOffsetT(len(v)) @@ -326,20 +347,38 @@ func (b *Builder) CreateByteVector(v []byte) UOffsetT { return b.EndVector(len(v)) } -func (b *Builder) notNested() { - // Check that no other objects are being built while making this - // object. If not, panic: - if b.insideObject { - panic("non-inline data write inside of object") +func (b *Builder) assertNested() { + // If you get this assert, you're in an object while trying to write + // data that belongs outside of an object. + // To fix this, write non-inline data (like vectors) before creating + // objects. + if !b.nested { + panic("Incorrect creation order: must be inside object.") } } -func (b *Builder) nested(obj UOffsetT) { - // Structs are always stored inline, so need to be created right - // where they are used. You'll get this panic if you created it - // elsewhere: - if obj != b.Offset() { - panic("inline data write outside of object") +func (b *Builder) assertNotNested() { + // If you hit this, you're trying to construct a Table/Vector/String + // during the construction of its parent table (between the MyTableBuilder + // and builder.Finish()). + // Move the creation of these sub-objects to above the MyTableBuilder to + // not get this assert. + // Ignoring this assert may appear to work in simple cases, but the reason + // it is here is that storing objects in-line may cause vtable offsets + // to not fit anymore. It also leads to vtable duplication. + if b.nested { + panic("Incorrect creation order: object must not be nested.") + } +} + +func (b *Builder) assertFinished() { + // If you get this assert, you're attempting to get access a buffer + // which hasn't been finished yet. Be sure to call builder.Finish() + // with your root table. + // If you really need to access an unfinished buffer, use the Bytes + // buffer directly. + if !b.finished { + panic("Incorrect use of FinishedBytes(): must call 'Finish' first.") } } @@ -483,7 +522,10 @@ func (b *Builder) PrependUOffsetTSlot(o int, x, d UOffsetT) { // In generated code, `d` is always 0. func (b *Builder) PrependStructSlot(voffset int, x, d UOffsetT) { if x != d { - b.nested(x) + b.assertNested() + if x != b.Offset() { + panic("inline data write outside of object") + } b.Slot(voffset) } } @@ -495,8 +537,10 @@ func (b *Builder) Slot(slotnum int) { // Finish finalizes a buffer, pointing to the given `rootTable`. func (b *Builder) Finish(rootTable UOffsetT) { + b.assertNotNested() b.Prep(b.minalign, SizeUOffsetT) b.PrependUOffsetT(rootTable) + b.finished = true } // vtableEqual compares an unwritten vtable to a written vtable. diff --git a/tests/go_test.go b/tests/go_test.go index 4eb8d3e6a..9d94277f7 100644 --- a/tests/go_test.go +++ b/tests/go_test.go @@ -70,10 +70,10 @@ func TestAll(t *testing.T) { // Verify that panics are raised during exceptional conditions: CheckNotInObjectError(t.Fatalf) - CheckObjectIsNestedError(t.Fatalf) CheckStringIsNestedError(t.Fatalf) CheckByteStringIsNestedError(t.Fatalf) CheckStructIsNotInlineError(t.Fatalf) + CheckFinishedBytesError(t.Fatalf) // Verify that using the generated Go code builds a buffer without // returning errors: @@ -1113,20 +1113,6 @@ func CheckNotInObjectError(fail func(string, ...interface{})) { b.EndObject() } -// CheckObjectIsNestedError verifies that an object can not be created inside -// another object. -func CheckObjectIsNestedError(fail func(string, ...interface{})) { - b := flatbuffers.NewBuilder(0) - b.StartObject(0) - defer func() { - r := recover() - if r == nil { - fail("expected panic in CheckObjectIsNestedError") - } - }() - b.StartObject(0) -} - // CheckStringIsNestedError verifies that a string can not be created inside // another object. func CheckStringIsNestedError(fail func(string, ...interface{})) { @@ -1169,6 +1155,20 @@ func CheckStructIsNotInlineError(fail func(string, ...interface{})) { b.PrependStructSlot(0, 1, 0) } +// CheckFinishedBytesError verifies that `FinishedBytes` panics if the table +// is not finished. +func CheckFinishedBytesError(fail func(string, ...interface{})) { + b := flatbuffers.NewBuilder(0) + + defer func() { + r := recover() + if r == nil { + fail("expected panic in CheckFinishedBytesError") + } + }() + b.FinishedBytes() +} + // CheckDocExample checks that the code given in FlatBuffers documentation // is syntactically correct. func CheckDocExample(buf []byte, off flatbuffers.UOffsetT, fail func(string, ...interface{})) {