write: use Cow for Section::data (gimli-rs#370)

crates/examples/src/bin/objcopy.rs

@@ -70,7 +70,7 @@ fn main() {
         if out_section.is_bss() {
             out_section.append_bss(in_section.size(), in_section.align());
         } else {
-            out_section.set_data(in_section.data().unwrap().into(), in_section.align());
+            out_section.set_data(in_section.data().unwrap(), in_section.align());
         }
         out_section.flags = in_section.flags();
         out_sections.insert(in_section.index(), section_id);

src/write/coff.rs

@@ -23,7 +23,7 @@ struct SymbolOffsets {
     aux_count: u8,
 }
 
-impl Object {
+impl<'a> Object<'a> {
     pub(crate) fn coff_section_info(
         &self,
         section: StandardSection,
@@ -589,7 +589,7 @@ impl Object {
                         length: U32Bytes::new(LE, section.size as u32),
                         number_of_relocations: U16Bytes::new(LE, section.relocations.len() as u16),
                         number_of_linenumbers: U16Bytes::default(),
-                        check_sum: U32Bytes::new(LE, checksum(section.data.as_slice())),
+                        check_sum: U32Bytes::new(LE, checksum(section.data())),
                         number: U16Bytes::new(
                             LE,
                             section_offsets[section_index].associative_section,

src/write/elf/object.rs

@@ -27,7 +27,7 @@ struct SymbolOffsets {
     str_id: Option<StringId>,
 }
 
-impl Object {
+impl<'a> Object<'a> {
     pub(crate) fn elf_section_info(
         &self,
         section: StandardSection,

src/write/macho.rs

@@ -22,7 +22,7 @@ struct SymbolOffsets {
     str_id: Option<StringId>,
 }
 
-impl Object {
+impl<'a> Object<'a> {
     pub(crate) fn macho_set_subsections_via_symbols(&mut self) {
         let flags = match self.flags {
             FileFlags::MachO { flags } => flags,

src/write/mod.rs

@@ -1,5 +1,6 @@
 //! Interface for writing object files.
 
+use std::borrow::Cow;
 use std::boxed::Box;
 use std::collections::HashMap;
 use std::string::String;
@@ -45,11 +46,11 @@ pub type Result<T> = result::Result<T, Error>;
 
 /// A writable object file.
 #[derive(Debug)]
-pub struct Object {
+pub struct Object<'a> {
     format: BinaryFormat,
     architecture: Architecture,
     endian: Endianness,
-    sections: Vec<Section>,
+    sections: Vec<Section<'a>>,
     standard_sections: HashMap<StandardSection, SectionId>,
     symbols: Vec<Symbol>,
     symbol_map: HashMap<Vec<u8>, SymbolId>,
@@ -63,9 +64,9 @@ pub struct Object {
     tlv_bootstrap: Option<SymbolId>,
 }
 
-impl Object {
+impl<'a> Object<'a> {
     /// Create an empty object file.
-    pub fn new(format: BinaryFormat, architecture: Architecture, endian: Endianness) -> Object {
+    pub fn new(format: BinaryFormat, architecture: Architecture, endian: Endianness) -> Object<'a> {
         Object {
             format,
             architecture,
@@ -124,16 +125,26 @@ impl Object {
 
     /// Get the section with the given `SectionId`.
     #[inline]
-    pub fn section(&self, section: SectionId) -> &Section {
+    pub fn section(&self, section: SectionId) -> &Section<'a> {
         &self.sections[section.0]
     }
 
     /// Mutably get the section with the given `SectionId`.
     #[inline]
-    pub fn section_mut(&mut self, section: SectionId) -> &mut Section {
+    pub fn section_mut(&mut self, section: SectionId) -> &mut Section<'a> {
         &mut self.sections[section.0]
     }
 
+    /// Set the data for an existing section.
+    ///
+    /// Must not be called for sections that already have data, or that contain uninitialized data.
+    pub fn set_section_data<T>(&mut self, section: SectionId, data: T, align: u64)
+    where
+        T: Into<Cow<'a, [u8]>>,
+    {
+        self.sections[section.0].set_data(data, align)
+    }
+
     /// Append data to an existing section. Returns the section offset of the data.
     pub fn append_section_data(&mut self, section: SectionId, data: &[u8], align: u64) -> u64 {
         self.sections[section.0].append_data(data, align)
@@ -168,7 +179,7 @@ impl Object {
             kind,
             size: 0,
             align: 1,
-            data: Vec::new(),
+            data: Cow::Borrowed(&[]),
             relocations: Vec::new(),
             symbol: None,
             flags: SectionFlags::None,
@@ -502,7 +513,7 @@ impl Object {
         relocation: &Relocation,
         addend: i64,
     ) -> Result<()> {
-        let data = &mut self.sections[section.0].data;
+        let data = self.sections[section.0].data_mut();
         let offset = relocation.offset as usize;
         match relocation.size {
             32 => data.write_at(offset, &U32::new(self.endian, addend as u32)),
@@ -627,20 +638,20 @@ pub struct SectionId(usize);
 
 /// A section in an object file.
 #[derive(Debug)]
-pub struct Section {
+pub struct Section<'a> {
     segment: Vec<u8>,
     name: Vec<u8>,
     kind: SectionKind,
     size: u64,
     align: u64,
-    data: Vec<u8>,
+    data: Cow<'a, [u8]>,
     relocations: Vec<Relocation>,
     symbol: Option<SymbolId>,
     /// Section flags that are specific to each file format.
     pub flags: SectionFlags,
 }
 
-impl Section {
+impl<'a> Section<'a> {
     /// Try to convert the name to a utf8 string.
     #[inline]
     pub fn name(&self) -> Option<&str> {
@@ -662,32 +673,36 @@ impl Section {
     /// Set the data for a section.
     ///
     /// Must not be called for sections that already have data, or that contain uninitialized data.
-    pub fn set_data(&mut self, data: Vec<u8>, align: u64) {
+    pub fn set_data<T>(&mut self, data: T, align: u64)
+    where
+        T: Into<Cow<'a, [u8]>>,
+    {
         debug_assert!(!self.is_bss());
         debug_assert_eq!(align & (align - 1), 0);
         debug_assert!(self.data.is_empty());
-        self.size = data.len() as u64;
-        self.data = data;
+        self.data = data.into();
+        self.size = self.data.len() as u64;
         self.align = align;
     }
 
     /// Append data to a section.
     ///
     /// Must not be called for sections that contain uninitialized data.
-    pub fn append_data(&mut self, data: &[u8], align: u64) -> u64 {
+    pub fn append_data(&mut self, append_data: &[u8], align: u64) -> u64 {
         debug_assert!(!self.is_bss());
         debug_assert_eq!(align & (align - 1), 0);
         if self.align < align {
             self.align = align;
         }
         let align = align as usize;
-        let mut offset = self.data.len();
+        let data = self.data.to_mut();
+        let mut offset = data.len();
         if offset & (align - 1) != 0 {
             offset += align - (offset & (align - 1));
-            self.data.resize(offset, 0);
+            data.resize(offset, 0);
         }
-        self.data.extend_from_slice(data);
-        self.size = self.data.len() as u64;
+        data.extend_from_slice(append_data);
+        self.size = data.len() as u64;
         offset as u64
     }
 
@@ -722,7 +737,7 @@ impl Section {
     /// This requires that the section is not a bss section.
     pub fn data_mut(&mut self) -> &mut [u8] {
         debug_assert!(!self.is_bss());
-        &mut self.data
+        self.data.to_mut()
     }
 }