Introduce UnsafeFlag to manage disabling ArrayData validation (apache#7027)

* Introduce UnsafeFlag to manage disabling validation

* fix docs

Author: alamb

arrow-data/src/data.rs

@@ -28,6 +28,7 @@ use std::mem;
 use std::ops::Range;
 use std::sync::Arc;
 
+use crate::data::private::UnsafeFlag;
 use crate::{equal, validate_binary_view, validate_string_view};
 
 #[inline]
@@ -255,6 +256,10 @@ impl ArrayData {
         buffers: Vec<Buffer>,
         child_data: Vec<ArrayData>,
     ) -> Self {
+        let mut skip_validation = UnsafeFlag::new();
+        // SAFETY: caller responsible for ensuring data is valid
+        skip_validation.set(true);
+
         ArrayDataBuilder {
             data_type,
             len,
@@ -265,8 +270,7 @@ impl ArrayData {
             buffers,
             child_data,
             align_buffers: false,
-            // SAFETY: caller responsible for ensuring data is valid
-            skip_validation: true,
+            skip_validation,
         }
         .build()
         .unwrap()
@@ -1779,6 +1783,36 @@ impl PartialEq for ArrayData {
     }
 }
 
+mod private {
+    /// A boolean flag that cannot be mutated outside of unsafe code.
+    ///
+    /// Defaults to a value of false.
+    ///
+    /// This structure is used to enforce safety in the [`ArrayDataBuilder`]
+    ///
+    /// [`ArrayDataBuilder`]: super::ArrayDataBuilder
+    #[derive(Debug)]
+    pub struct UnsafeFlag(bool);
+
+    impl UnsafeFlag {
+        /// Creates a new `UnsafeFlag` with the value set to `false`
+        #[inline]
+        pub const fn new() -> Self {
+            Self(false)
+        }
+
+        #[inline]
+        pub unsafe fn set(&mut self, val: bool) {
+            self.0 = val;
+        }
+
+        #[inline]
+        pub fn get(&self) -> bool {
+            self.0
+        }
+    }
+}
+
 /// Builder for [`ArrayData`] type
 #[derive(Debug)]
 pub struct ArrayDataBuilder {
@@ -1803,7 +1837,7 @@ pub struct ArrayDataBuilder {
     /// This flag can only be set to true using `unsafe` APIs. However, once true
     /// subsequent calls to `build()` may result in undefined behavior if the data
     /// is not valid.
-    skip_validation: bool,
+    skip_validation: UnsafeFlag,
 }
 
 impl ArrayDataBuilder {
@@ -1820,7 +1854,7 @@ impl ArrayDataBuilder {
             buffers: vec![],
             child_data: vec![],
             align_buffers: false,
-            skip_validation: false,
+            skip_validation: UnsafeFlag::new(),
         }
     }
 
@@ -1957,7 +1991,7 @@ impl ArrayDataBuilder {
         }
 
         // SAFETY: `skip_validation` is only set to true using `unsafe` APIs
-        if !skip_validation || cfg!(feature = "force_validate") {
+        if !skip_validation.get() || cfg!(feature = "force_validate") {
             data.validate_data()?;
         }
         Ok(data)
@@ -2003,7 +2037,7 @@ impl ArrayDataBuilder {
     /// If validation is skipped, the buffers must form a valid Arrow array,
     /// otherwise undefined behavior will result
     pub unsafe fn skip_validation(mut self, skip_validation: bool) -> Self {
-        self.skip_validation = skip_validation;
+        self.skip_validation.set(skip_validation);
         self
     }
 }
@@ -2020,7 +2054,7 @@ impl From<ArrayData> for ArrayDataBuilder {
             null_bit_buffer: None,
             null_count: None,
             align_buffers: false,
-            skip_validation: false,
+            skip_validation: UnsafeFlag::new(),
         }
     }
 }