base/slab.h: remove exceptions from constructor

Issue #5245

repos/base/include/base/slab.h

@@ -95,10 +95,6 @@ class Genode::Slab : public Allocator
 		 * block that is used for the first couple of allocations,
 		 * especially for the allocation of the second slab
 		 * block.
-		 * 
-		 * \throw Out_of_ram
-		 * \throw Out_of_caps
-		 * \throw Allocator::Denied  failed to obtain initial slab block
 		 */
 		Slab(size_t slab_size, size_t block_size, void *initial_sb,
 		     Allocator *backing_store = 0);
@@ -143,7 +139,6 @@ class Genode::Slab : public Allocator
 		 */
 		Allocator *backing_store() { return _backing_store; }
 
-
 		/**
 		 * Free memory of empty slab blocks
 		 */

repos/base/src/lib/base/slab.cc

@@ -221,26 +221,10 @@ Slab::Slab(size_t slab_size, size_t block_size, void *initial_sb,
 	                   / (_slab_size + sizeof(Entry) + 1)),
 	_initial_sb((Block *)initial_sb),
 	_nested(false),
-	_curr_sb((Block *)initial_sb),
 	_backing_store(backing_store)
 {
 	static_assert(sizeof(Slab::Block) <= overhead_per_block());
 	static_assert(sizeof(Slab::Entry) <= overhead_per_entry());
-
-	/* if no initial slab block was specified, try to get one */
-	if (!_curr_sb && _backing_store)
-		_new_slab_block().with_result(
-			[&] (Block *sb) { _curr_sb = sb; },
-			[&] (Alloc_error error) {
-				Allocator::throw_alloc_error(error); });
-
-	if (!_curr_sb)
-		throw Allocator::Denied();
-
-	/* init first slab block */
-	construct_at<Block>(_curr_sb, *this);
-	_total_avail = _entries_per_block;
-	_num_blocks  = 1;
 }
 
 
@@ -280,6 +264,9 @@ Slab::New_slab_block_result Slab::_new_slab_block()
 
 void Slab::_release_backing_store(Block *block)
 {
+	if (!block)
+		return;
+
 	if (block->avail() != _entries_per_block)
 		error("freeing non-empty slab block");
 
@@ -375,6 +362,24 @@ Allocator::Alloc_result Slab::try_alloc(size_t size)
 		return Alloc_error::DENIED;
 	}
 
+	/* init very first block */
+	if (!_curr_sb) {
+		Alloc_error error = Alloc_error::DENIED;
+		if (_initial_sb)
+			_curr_sb = _initial_sb;
+		else
+			_new_slab_block().with_result(
+				[&] (Block *sb) { _curr_sb = sb; },
+				[&] (Alloc_error e) { error = e; });
+
+		if (!_curr_sb)
+			return error;
+
+		construct_at<Block>(_curr_sb, *this);
+		_total_avail = _entries_per_block;
+		_num_blocks  = 1;
+	}
+
 	/*
 	 * If we run out of slab, we need to allocate a new slab block. For the
 	 * special case that this block is allocated using the allocator that by